#493 Upgrade to NUnit 4.x and TestCentric 2.x

[Maettu]

This is a follow-up to the Upgrade to NUnit 3.x covering all remaining input and issues:

To be checked with TestCentric 2.x:

• ArgumentNullException parameter tests at TestCentric.Gui.Model.TestModel.RunTests(TestSelection tests)
• NUnit isn't able to deal with PublicSign
• TestCentric fails to load .nunit project file that contains parent relative path

To be checked with NUnit 4.x:

• Provide a way to ignore a failed assertion at TestExecutionContextEx
• Missing possibility to retrieve a test's resulting ParallelScope at TestContextEx and TestExecutionContextEx
• Control which tests may run in parallel with which other tests
• Check on type safe TestCaseData

New NUnit 4.x features:

• Add DefaultConstraint
• Add support for Test Cancellation
• Support tuples in inline data