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/systemd-resolved/cmd/resolver-tools/guard_test.go

@@ -185,7 +185,7 @@ func TestTheVerbsOnTheMachine(t *testing.T) {
 	defer cancel()
 	sock := filepath.Join(g.Dir, "verbs.sock")
 	go func() { _ = g.Serve(ctx, sock) }()
-	for i := 0; i < 100; i++ {
+	for i := 0; i < 3000; i++ {
 		if _, err := os.Stat(sock); err == nil {
 			break
 		}
@@ -233,7 +233,7 @@ func TestTheVerbsOnTheMachine(t *testing.T) {
 	select {
 	case <-g.wake:
 		g.Tick()
-	case <-time.After(time.Second):
+	case <-time.After(30 * time.Second):
 		t.Fatal("taking the write did not wake the guard")
 	}
 	if contentOf(t, g.Path) != meshFile {