Add tests for ProperContains and ProperIn

[antvaset]

Adding more tests for ProperContains and ProperIn operators to cover more edge cases for the list overload.

Adjusting 4 existing tests to make it clear and make sure that ProperContainsNullRightFalse and ProperContainsNullRightTrue compile to ProperContains(List<T>, T) and that ProperInNullRightFalse and ProperInNullRightTrue compile to ProperIn(T, List<T>), matching the test name/group. Without null as String, the args could be resolved as List<T>, List<T> and compile to ProperIncludes(List<T>, List<T>) and ProperIncludedIn(List<T>, List<T>).

[antvaset]

This PR cannot be reopened, so I had to create a new one - see #48.