Add support for Test Cancellation

[manfred-brands]

This is to augment the existing TimeoutAttribute
The latter which has problems on .NET Core version of NUnit #4021

[goblinmaks]

@manfred-brands Have chance it to be merged as soonest time ?

[manfred-brands]

@goblinmaks I have managed to fix the issues I had under Linux/MacOS (using Mono) and marked the PR as ready for review.