Make it possible to add coverage to the existing state

[loewexy]

Currently when running any test with coverage VScode will discard the current coverage information and replace it by the one gathered with the new test.

If would be helpful if there was a way to add the coverage to the currently collected data. With that one could write an additional test and see how the coverage improved. Currently to get a new information one has to re-run the complete test suite. I have tests suites running for 30 minutes and more, therefore this is very inconvenient.

That behavior could be the default, and there could be a button to reset the info. Or a mode that can be activated, I am not sure what best fits the general design idea. For me this does not make a large difference.