Removed Moryx.TestTools.NUnit and used NUnit.Constraints

[dbeuchler]

Moryx.TestTools.NUnit was introduced for providing custom asserts for NUnit and providing the CallerArgumentExpression. No migration guide entry because it was not a package.

NUnit 4 includes that by default: https://docs.nunit.org/articles/nunit-analyzers/NUnit2050.html

In order to get better failure messages, NUnit4 uses CallerArgumentExpression to include the expression passed in for the actual and constraint parameters. These are parameters automatically supplied by the compiler.

For better exceptions, all features of NUnit.Constraints can be used.