[gdb/testsuite] Use early-out style in gdb.base/break-probes.exp

Reduce indentation and improve readability in test-case
gdb.base/break-probes.exp by replacing:
...
if { <cond> } {
  <lots-of-code>
}
...
with:
...
if { ! <cond> } {
  return -1
}
<lots-of-code>
...

Tested on x86_64-linux.

gdb/testsuite/gdb.base/break-probes.exp

@@ -60,34 +60,36 @@ gdb_test_multiple "bt" $test {
     }
 }
 
-if { $using_probes } {
-    set sysroot ""
-    set test "show sysroot"
-    gdb_test_multiple $test $test {
-	-re "The current system root is \"(.*)\"\..*${gdb_prompt} $" {
-	    set sysroot $expect_out(1,string)
-	}
+if { ! $using_probes } {
+    return -1
+}
+
+set sysroot ""
+set test "show sysroot"
+gdb_test_multiple $test $test {
+    -re "The current system root is \"(.*)\"\..*${gdb_prompt} $" {
+	set sysroot $expect_out(1,string)
     }
+}
 
-    # GDB strips "target:" from the start of filenames
-    # when operating on the local filesystem
-    regsub "^target:" "$sysroot" "(target:)?" sysroot
+# GDB strips "target:" from the start of filenames
+# when operating on the local filesystem
+regsub "^target:" "$sysroot" "(target:)?" sysroot
 
-    # Run til it loads our library
-    set test "run til our library loads"
-    set not_loaded_library 1
-    while { $not_loaded_library } {
-	set not_loaded_library 0
-	gdb_test_multiple "c" $test {
-	    -re "Inferior loaded $sysroot$binfile_lib\\M.*$gdb_prompt $" {
-		pass $test
-	    }
-	    -re "Stopped due to shared library event\\M.*$gdb_prompt $" {
-		set not_loaded_library 1
-	    }
+# Run til it loads our library
+set test "run til our library loads"
+set not_loaded_library 1
+while { $not_loaded_library } {
+    set not_loaded_library 0
+    gdb_test_multiple "c" $test {
+	-re "Inferior loaded $sysroot$binfile_lib\\M.*$gdb_prompt $" {
+	    pass $test
+	}
+	-re "Stopped due to shared library event\\M.*$gdb_prompt $" {
+	    set not_loaded_library 1
 	}
     }
-
-    # Call something to ensure that relocation occurred
-    gdb_test "call (int) foo(23)" "\\\$.* = 31.*\\\M.*"
 }
+
+# Call something to ensure that relocation occurred
+gdb_test "call (int) foo(23)" "\\\$.* = 31.*\\\M.*"