Investigate potential memory leak in TestRunCancellationToken

[Youssef1313]

TestRunCancellationToken maintains a list of registered callbacks.

One of the paths for the registration is to associate the global test run cancellation with TestContext CTS (i.e, if the global is cancelled, then the TestContext one is cancelled as well).

I think this means that all created TextContext will remain alive in memory as we don't unregister the callbacks any where.