Interactive debugging

[Neyts Zupan]

While working on a PoC that eventually became tesh, I realized we desperately need intereactive debugging.

It's super annoying to have some tests run for a few minutes, error out, and you have to manually run all commands to get to the failure point to debug it. Ideally, given --debug flag, tesh drops you into the session under test, so you can look around and see why the test failed.