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-mail-reader/cmd/mesh-mail-reader/batch_test.go

@@ -127,7 +127,7 @@ func TestThirtyWaitingMailsRunAsThreeBatchesAtLeast160sApart(t *testing.T) {
 	}()
 	select {
 	case <-done:
-	case <-time.After(5 * time.Second):
+	case <-time.After(30 * time.Second):
 		t.Fatal("RunBatches did not stop after its own 3rd batch and this test's cancel")
 	}