ktesting: fix potential log panic

If the goroutine happens to log after the test has already terminated,
testing.T.Log panics. We must ensure that the goroutine has stopped before
allowing the test to terminate.

test/utils/ktesting/contexthelper.go

@@ -48,11 +48,17 @@ func withTimeout(ctx context.Context, tb TB, timeout time.Duration, timeoutCause
 	cancelCtx, cancel := context.WithCancelCause(ctx)
 	after := time.NewTimer(timeout)
 	stopCtx, stop := context.WithCancel(ctx) // Only used internally, doesn't need a cause.
+	done := make(chan struct{})
 	tb.Cleanup(func() {
 		cancel(cleanupErr(tb.Name()))
 		stop()
+		// Wait for goroutine termination. This is important because
+		// otherwise the goroutine might log through tb *after* the
+		// test has already finished, which causes a panic.
+		<-done
 	})
 	go func() {
+		defer close(done)
 		select {
 		case <-stopCtx.Done():
 			after.Stop()