Fix flaky tests caused by SNS policy propagation delays

[tmasternak]

Some of the acceptance tests are written in such a way, that they can suffer from SNS access policy propagation delays. These are tests where more than one manual Subscribe operation is called:

• Remove_multiple_events_when_setting_wildcard_policy - fails with error System.InvalidOperationException : Process must exit before requested information can be determined.
• Should_still_deliver - fails with TestTimeout exception