Wait for the third run to start, not only for its take, so the capacity test does not race under a loaded suite

go/buildstep/pull_test.go

@@ -127,11 +127,8 @@ func TestPullHoldsAtMostItsCapacity(t *testing.T) {
 	}
 	release <- struct{}{}
 	waitFor(t, "the third taken", func() bool { return src.Waiting(delivery.AskDelivery) == 0 })
-	mu.Lock()
-	if started != 3 {
-		t.Errorf("started %d, want 3", started)
-	}
-	mu.Unlock()
+	// Taken from the source comes a moment before its run starts: wait for the start, not only the take.
+	waitFor(t, "the third started", func() bool { mu.Lock(); defer mu.Unlock(); return started == 3 })
 	close(release)
 	waitFor(t, "all acked", func() bool { a, _, _ := src.Fates(); return len(a) == 3 })
 }