[Chromecast] Fix CastAudioOutputStream::CloseWithoutStop test.

Close is not a synchronous call; allow the thread to finish before exiting.

Bug: b/135104876
Change-Id: I32c5f36264757100678d258e8508ec45f3341d49
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1656674
Commit-Queue: Alex Leung <alexleung@google.com>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Cr-Commit-Position: refs/heads/master@{#668648}

chromecast/media/audio/cast_audio_output_stream_unittest.cc

@@ -369,6 +369,7 @@ TEST_F(CastAudioOutputStreamTest, CloseWithoutStop) {
   RunThreadsUntilIdle();
 
   stream->Close();
+  RunThreadsUntilIdle();
 }
 
 TEST_F(CastAudioOutputStreamTest, CloseCancelsOpen) {