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/disk-load/cmd/disk-load/data_test.go

@@ -181,10 +181,10 @@ func readingOf(s *memStore, key string) *health.Reading {
 
 func waitFor(t *testing.T, done func() bool) {
 	t.Helper()
-	for end := time.Now().Add(5 * time.Second); time.Now().Before(end); time.Sleep(5 * time.Millisecond) {
+	for end := time.Now().Add(30 * time.Second); time.Now().Before(end); time.Sleep(5 * time.Millisecond) {
 		if done() {
 			return
 		}
 	}
-	t.Fatal("not done within 5 s")
+	t.Fatal("not done within 30 s")
 }