Make tests that wait for concurrent work wait on the event or a condition with a long deadline (issue 1814)

A test that slept a fixed time and then asserted what a goroutine, process or
watcher did failed under the checker's load. Each now waits for its positive
evidence (a started baseline, a call counted, a state reached) within 30 s, and
the concurrency-limit tests meet at a barrier instead of overlapping by sleep.

Claude-Session: https://claude.ai/code/session_01NcbgfJ9Gum47zkFKWRaovL

modules/screen-lock/cmd/screen-lock-tools/lock_test.go

@@ -170,7 +170,12 @@ func TestAnInhibitionTurnsIdleOffAndRestoresWhatWasThereWhenItEndsOrIsStopped(t
 	if err := cmd.Start(); err != nil {
 		t.Fatal(err)
 	}
-	time.Sleep(300 * time.Millisecond)
+	// stop it only once it has turned idle off: the trap is set before that, and a slow start must not be signalled early
+	for end := time.Now().Add(30 * time.Second); time.Now().Before(end); time.Sleep(10 * time.Millisecond) {
+		if b, _ := os.ReadFile(filepath.Join(dir, "log")); strings.Contains(string(b), "s off -dpms") {
+			break
+		}
+	}
 	_ = cmd.Process.Signal(os.Interrupt)
 	_ = cmd.Wait()
 	got, _ := os.ReadFile(filepath.Join(dir, "log"))