fix: #902 stabilize testCompletionErrorOnMissingContext

[ashakirin]

fix: #902 stabilize testCompletionErrorOnMissingContext with client retry logic

[Kehrlann]

@ashakirin the retry seems a bit overkill.

It seems we use .close() on the client with a try-with-resources. By default, this does .closeGracefully().subscribe(), which starts the closing process but doesn't wait.

Should we try and use StepVerifier.create(transport.closeGracefully()).verifyComplete(); instead?