testsuite: fix failure in gdb.threads/killed-outside.exp

Starting with commit

  commit 1da5d0e
  Date:   Tue Jan 4 08:02:24 2022 -0700

    Change how Python architecture and language are handled

we see a failure in gdb.threads/killed-outside.exp:

  ...
  Executing on target: kill -9 16622    (timeout = 300)
  builtin_spawn -ignore SIGHUP kill -9 16622
  continue
  Continuing.
  Couldn't get registers: No such process.
  (gdb) [Thread 0x7ffff77c2700 (LWP 16626) exited]

  Program terminated with signal SIGKILL, Killed.
  The program no longer exists.
  FAIL: gdb.threads/killed-outside.exp: prompt after first continue (timeout)

This is not a regression but a failure due to a change in GDB's
output.  Prior to the aforementioned commit, GDB has been printing the
"Couldn't get registers: No such process." message twice.  The second
one came from

  (top-gdb) bt
  #0  amd64_linux_nat_target::fetch_registers (this=0x555557f31440 <the_amd64_linux_nat_target>, regcache=0x555558805ce0, regnum=16) at /gdb-up/gdb/amd64-linux-nat.c:225
  #1  0x000055555640ac5f in target_ops::fetch_registers (this=0x555557d636d0 <the_thread_db_target>, arg0=0x555558805ce0, arg1=16) at /gdb-up/gdb/target-delegates.c:502
  #2  0x000055555641a647 in target_fetch_registers (regcache=0x555558805ce0, regno=16) at /gdb-up/gdb/target.c:3945
  #3  0x0000555556278e68 in regcache::raw_update (this=0x555558805ce0, regnum=16) at /gdb-up/gdb/regcache.c:587
  #4  0x0000555556278f14 in readable_regcache::raw_read (this=0x555558805ce0, regnum=16, buf=0x555558881950 "") at /gdb-up/gdb/regcache.c:601
  #5  0x00005555562792aa in readable_regcache::cooked_read (this=0x555558805ce0, regnum=16, buf=0x555558881950 "") at /gdb-up/gdb/regcache.c:690
  #6  0x000055555627965e in readable_regcache::cooked_read_value (this=0x555558805ce0, regnum=16) at /gdb-up/gdb/regcache.c:748
  #7  0x0000555556352a37 in sentinel_frame_prev_register (this_frame=0x555558181090, this_prologue_cache=0x5555581810a8, regnum=16) at /gdb-up/gdb/sentinel-frame.c:53
  #8  0x0000555555fa4773 in frame_unwind_register_value (next_frame=0x555558181090, regnum=16) at /gdb-up/gdb/frame.c:1235
  #9  0x0000555555fa420d in frame_register_unwind (next_frame=0x555558181090, regnum=16, optimizedp=0x7fffffffd570, unavailablep=0x7fffffffd574, lvalp=0x7fffffffd57c, addrp=0x7fffffffd580,
      realnump=0x7fffffffd578, bufferp=0x7fffffffd5b0 "") at /gdb-up/gdb/frame.c:1143
  #10 0x0000555555fa455f in frame_unwind_register (next_frame=0x555558181090, regnum=16, buf=0x7fffffffd5b0 "") at /gdb-up/gdb/frame.c:1199
  #11 0x00005555560178e2 in i386_unwind_pc (gdbarch=0x5555587c4a70, next_frame=0x555558181090) at /gdb-up/gdb/i386-tdep.c:1972
  #12 0x0000555555cd2b9d in gdbarch_unwind_pc (gdbarch=0x5555587c4a70, next_frame=0x555558181090) at /gdb-up/gdb/gdbarch.c:3007
  #13 0x0000555555fa3a5b in frame_unwind_pc (this_frame=0x555558181090) at /gdb-up/gdb/frame.c:948
  #14 0x0000555555fa7621 in get_frame_pc (frame=0x555558181160) at /gdb-up/gdb/frame.c:2572
  #15 0x0000555555fa7706 in get_frame_address_in_block (this_frame=0x555558181160) at /gdb-up/gdb/frame.c:2602
  #16 0x0000555555fa77d0 in get_frame_address_in_block_if_available (this_frame=0x555558181160, pc=0x7fffffffd708) at /gdb-up/gdb/frame.c:2665
  #17 0x0000555555fa5f8d in select_frame (fi=0x555558181160) at /gdb-up/gdb/frame.c:1890
  #18 0x0000555555fa5bab in lookup_selected_frame (a_frame_id=..., frame_level=-1) at /gdb-up/gdb/frame.c:1720
  #19 0x0000555555fa5e47 in get_selected_frame (message=0x0) at /gdb-up/gdb/frame.c:1810
  #20 0x0000555555cc9c6e in get_current_arch () at /gdb-up/gdb/arch-utils.c:848
  #21 0x000055555625b239 in gdbpy_before_prompt_hook (extlang=0x555557451f20 <extension_language_python>, current_gdb_prompt=0x555557f4d890 <top_prompt+16> "(gdb) ")
      at /gdb-up/gdb/python/python.c:1063
  #22 0x0000555555f7cfbb in ext_lang_before_prompt (current_gdb_prompt=0x555557f4d890 <top_prompt+16> "(gdb) ") at /gdb-up/gdb/extension.c:922
  #23 0x0000555555f7d442 in std::_Function_handler<void (char const*), void (*)(char const*)>::_M_invoke(std::_Any_data const&, char const*&&) (__functor=...,
      __args#0=@0x7fffffffd900: 0x555557f4d890 <top_prompt+16> "(gdb) ") at /usr/include/c++/7/bits/std_function.h:316
  #24 0x0000555555f752dd in std::function<void (char const*)>::operator()(char const*) const (this=0x55555817d838, __args#0=0x555557f4d890 <top_prompt+16> "(gdb) ")
      at /usr/include/c++/7/bits/std_function.h:706
  #25 0x0000555555f75100 in gdb::observers::observable<char const*>::notify (this=0x555557f49060 <gdb::observers::before_prompt>, args#0=0x555557f4d890 <top_prompt+16> "(gdb) ")
      at /gdb-up/gdb/../gdbsupport/observable.h:150
  #26 0x0000555555f736dc in top_level_prompt () at /gdb-up/gdb/event-top.c:444
  #27 0x0000555555f735ba in display_gdb_prompt (new_prompt=0x0) at /gdb-up/gdb/event-top.c:411
  #28 0x00005555564611a7 in tui_on_command_error () at /gdb-up/gdb/tui/tui-interp.c:205
  #29 0x0000555555c2173f in std::_Function_handler<void (), void (*)()>::_M_invoke(std::_Any_data const&) (__functor=...) at /usr/include/c++/7/bits/std_function.h:316
  #30 0x0000555555e10c20 in std::function<void ()>::operator()() const (this=0x5555580f9028) at /usr/include/c++/7/bits/std_function.h:706
  #31 0x0000555555e10973 in gdb::observers::observable<>::notify() const (this=0x555557f48d20 <gdb::observers::command_error>) at /gdb-up/gdb/../gdbsupport/observable.h:150
  #32 0x00005555560e9b3f in start_event_loop () at /gdb-up/gdb/main.c:438
  #33 0x00005555560e9bcc in captured_command_loop () at /gdb-up/gdb/main.c:481
  #34 0x00005555560eb616 in captured_main (data=0x7fffffffddd0) at /gdb-up/gdb/main.c:1348
  #35 0x00005555560eb67c in gdb_main (args=0x7fffffffddd0) at /gdb-up/gdb/main.c:1363
  #36 0x0000555555c1b6b3 in main (argc=12, argv=0x7fffffffded8) at /gdb-up/gdb/gdb.c:32

Commit 1da5d0e eliminated the call to 'get_current_arch'
in 'gdbpy_before_prompt_hook'.  Hence, the second instance of
"Couldn't get registers: No such process." does not appear anymore.

Fix the failure by updating the regular expression in the test.

gdb/testsuite/gdb.threads/killed-outside.exp

@@ -43,9 +43,9 @@ set no_longer_exists_msg "The program no longer exists\."
 set not_being_run_msg "The program is not being run\."
 
 gdb_test_multiple "continue" "prompt after first continue" {
-    -re "Continuing\.\r\n$no_such_process_msg\r\n$no_such_process_msg\r\n$gdb_prompt " {
+    -re "Continuing\.\r\n$no_such_process_msg\r\n$gdb_prompt " {
 	pass $gdb_test_name
-	# Two times $no_such_process_msg.  The bug condition was triggered, go
+	# Saw $no_such_process_msg.  The bug condition was triggered, go
 	# check for it.
 	gdb_test_multiple "" "messages" {
 	    -re ".*$killed_msg.*$no_longer_exists_msg\r\n" {