Increase mutation score

[jnyrup]

I installed a new version of Stryker, gave it a spin and added test cases for some low-hanging fruit.

Highlights:

• TypeSelectorAssertions.NotBeDecoratedWithOrInherit(predicate) only had a test verifying null check of the predicate, but no positive/negative test of the logic.
• [Not]MatchRegex would include two failure messages if used inside an AssertionScope
• No tests for NonGenericCollectionAssertions.HaveCount(predicate)
• No tests for NotContainKeys and NotContainValues when there is a single unexpected key/value

For anyone interessted here's the mutation report

Part of #1068

[dennisdoomen]

Would be cool to have the Stryker.NET output as build output at some point in the future.