Implement test runner cancellation

[IgorFedchenko]

TestExecutor has Cancel method used by dotnet test to perform cancellation. VS will use it to stop tests too.

It is no-op now, but should cancel all running nodes (I guess by killing processes) and stop MNTR.

[Aaronontheweb]

@Arkatufus how is this handled currently in the NuGet package we've published?

[Aaronontheweb]

To implement this:

• Iterate through nodeProcesses and call Process.Kill on each of them.
• Write the appropriate "test run cancelled" message to the test output.