it appears that there is no way to test diagnostics, and this does seem very similar to Test::Tester which can do that.
Yes, at the time I wrote TB2::Tester diagnostics were not proper events and could not be captured. As of 2.00_07 they are. So it should be possible to make them testable via TB2::Tester. I'd love to see that patch.
If you want to help TB2, working on TB2::Tester is a great way to do that. I haven't given it a lot of thought and it's a fairly discrete sub-project to take on.