Fix race condition in async_read_stream_test. (#2548)
Fix race condition in async_read_stream_test. (#2548)

The test for "read 3 messages and then an error" was a bit optimistic:
sometimes all three messages and the error are coalesced and then the
client just sees the error. Add some synchronization to make sure this
does not happen.

googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp