Update .NET test to NUnit 4 and FluentAssertions #408

Job	Run time
test-netstandard	33s
test-unity (2021.3.27f1)	3m 15s
inspectcode	1m 33s
test-unity (2022.3.2f1)	3m 56s
test-mutation-gherkin / stryker	3m 1s
test-mutation-core / stryker	3m 31s
	15m 49s

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