net5: Set up CI to run tests on both netcore3 and net5

[zkat]

In order to properly support net5 and make sure all our relevant code works on that platform, we should start testing on net5 going forward -- but since we're already testing on netcore3, and we need to keep supporting that one, that means we'll need to run the tests once on each of the two platforms going forward. Unfortunate, but I think necessary.

[zkat]

@aortiz-msft Hey Arturo -- this is that issue I mentioned that someone else outside the xplat team might be able to help us with, since we ultimately want to do this on dev for other reasons.