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/mounts/cmd/mounts/failed_test.go

@@ -126,12 +126,12 @@ func TestTheActingProcessClearsBetweenItsLooks(t *testing.T) {
 	go func() { RunApply(ctx, a); close(done) }()
 	defer func() { cancel(); <-done }()
 
-	if !waitFor(t, 2*time.Second, func() bool { return f.called("systemctl enable --now srv-library.automount") }) {
+	if !waitFor(t, 30*time.Second, func() bool { return f.called("systemctl enable --now srv-library.automount") }) {
 		t.Fatal("the first look did not arm the pair")
 	}
 	// The server is away and someone opened the mount point: the mount failed after the look.
 	f.setUnit("srv-library.mount", map[string]string{"ActiveState": "failed", "Result": "timeout"})
-	if !waitFor(t, 2*time.Second, func() bool { return f.called("systemctl reset-failed srv-library.mount") }) {
+	if !waitFor(t, 30*time.Second, func() bool { return f.called("systemctl reset-failed srv-library.mount") }) {
 		t.Fatal("a mount that failed between looks, its server away, is cleared before the next look")
 	}
 }