Fix flaky PubSub test.
Fix flaky PubSub test.

The Eventually() code in https://github.com/GoogleCloudPlatform/dotnet-docs-samples/pull/462 had no effect because the exception thrown is an AggregateException.  So handle AggregateExceptions too.

Change-Id: Ibc93775400b70ca3cb443f83df3341e5ae117c7f

jsimonweb/csharp-docs-samples,jsimonweb/csharp-docs-samples,GoogleCloudPlatform/dotnet-docs-samples,GoogleCloudPlatform/dotnet-docs-samples,jsimonweb/csharp-docs-samples,GoogleCloudPlatform/dotnet-docs-samples,GoogleCloudPlatform/dotnet-docs-samples,jsimonweb/csharp-docs-samples