t7900: fix host-dependent behaviour when testing git-maintenance(1)

We have recently added a new test to t7900 that exercises whether
git-maintenance(1) fails as expected when the "schedule.lock" file
exists. The test depends on whether or not the host has the required
executables present to schedule maintenance tasks in the first place,
like systemd or launchctl -- if not, the test fails with an unrelated
error before even checking for the lock file. This fails for example in
our CI systems, where macOS images do not have launchctl available.

Fix this issue by creating a stub systemctl(1) binary and using the
systemd scheduler.

Reported-by: Jeff King <[email protected]>
Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

t/t7900-maintenance.sh

@@ -1012,10 +1012,15 @@ test_expect_success 'repacking loose objects is quiet' '
 '
 
 test_expect_success 'maintenance aborts with existing lock file' '
-	test_when_finished "rm -rf repo" &&
+	test_when_finished "rm -rf repo script" &&
+	mkdir script &&
+	write_script script/systemctl <<-\EOF &&
+	true
+	EOF
+
 	git init repo &&
 	: >repo/.git/objects/schedule.lock &&
-	test_must_fail git -C repo maintenance start 2>err &&
+	test_must_fail env PATH="$PWD/script:$PATH" git -C repo maintenance start --scheduler=systemd 2>err &&
 	test_grep "Another scheduled git-maintenance(1) process seems to be running" err
 '