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/mesh-files/cmd/mesh-files/reads_test.go

@@ -198,7 +198,7 @@ func TestOnlyATextFileIsRead(t *testing.T) {
 		if err == nil || !strings.Contains(err.Error(), "fifo") {
 			t.Errorf("a fifo: %v", err)
 		}
-	case <-time.After(5 * time.Second):
+	case <-time.After(30 * time.Second):
 		t.Fatal("a read of a fifo waits for a writer")
 	}
 }