Converted AsyncFunctionAssertions into real base class

[lg2de]

As listed in #1677 and started in #1972, I converted AsyncFunctionAssertions into "real" base class (protected constructor).
Further, I moved some methods from base class into concrete class to prevent from overwriting (using new keyword).

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[github-actions]

Qodana for .NET

It seems all right 👌

No new problems were found according to the checks applied

💡 Qodana analysis was run in the pull request mode: only the changed files were checked

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[jnyrup]

Great to get rid of some new modifiers 🧹

[lg2de]

Does anybody know why the Qodana build fails?

[ITaluone]

Fix Qodana complaints?

[lg2de]

Fix Qodana complaints?

I would like to ask @dennisdoomen . The code follows the single return rule, Dennis requested in the past.

[dennisdoomen]

I would like to ask @dennisdoomen . The code follows the single return rule, Dennis requested in the past.

I think we can safely remove that rule. It's too rigid to always apply. I'm fine if you update the qodana.yaml as part of this PR.