[Experimental] Add TestContext.Current

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Fix #6137

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@Youssef1313 I was also wondering if we want to take this as an opportunity to introduce different types/interfaces for the various stages of the context? The idea being that we could offer different state based on the stage (e.g. after test runs, we know the result, after all tests of a class run we know all results...)

[Youssef1313]

@Youssef1313 I was also wondering if we want to take this as an opportunity to introduce different types/interfaces for the various stages of the context? The idea being that we could offer different state based on the stage (e.g. after test runs, we know the result, after all tests of a class run we know all results...)

That seems like a much bigger piece of work. We definitely can do given some design proposal.