fixed - InternalExpectAsync does not await actionAsync() - causing ac…

[brah-mcdude]

…tionAsync to run as a detached task #5537

Fixes #
InternalExpectAsync does not await actionAsync() - causing actionAsync to run as a detached task #5537

Changes

1. added a test ExpectAsync_should_await_actionAsync that reproduced the issue by failing and stopped failing when fix was applied.
2. the actual fix: added async and await.

Please provide a brief description of the changes here.

1. the issue was that a test that should have failed, was not failing.
2. so I created test that reproduces this issue - a test that fails - when a failed assertion was not failing (a false positive).
3. once I created the test (RED), I fixed the issue (GREEN).
4. done

Include data from before / after this change here.

Before:

 Akka.TestKit.Tests.Xunit2.TestEventListenerTests.CustomEventFilterWarningTests.ExpectAsync_should_await_actionAsync
   Source: AllTestForEventFilterBase.cs line 180
   Duration: 3.6 sec

  Message: 
Assert.Throws() Failure
Expected: typeof(Akka.TestKit.Xunit2.Internals.AkkaEqualException)
Actual:   (No exception was thrown)

  Stack Trace: 
AllTestForEventFilterBase`1.ExpectAsync_should_await_actionAsync() line 183
--- End of stack trace from previous location ---

After:

 Akka.TestKit.Tests.Xunit2.TestEventListenerTests.CustomEventFilterWarningTests.ExpectAsync_should_await_actionAsync
   Source: AllTestForEventFilterBase.cs line 180
   Duration: 450 ms

[Aaronontheweb]

LGTM

[brah-mcdude]

Just for the record, here is the original pull request where I described and reproduced the issue:
https://github.com/akkadotnet/akka.net/pull/5529/files