Fix mypy_test if mypy returns negative exit code

[srittau]

No description provided.

[AlexWaygood]

Things I like here:

• The existing behaviour of propagating the "maximum" returncode didn't make much sense; this PR fixes it
• I love the new MypyResult enum

I'm not so sure about the changes to the TestResults class, though. Something that struck me as I was reviewing #10724 was that it's a bit weird that we use the same TestResults class to represent "the results of running mypy on one stubs package" and "the results of running mypy_test.py on the whole of typeshed". Those are conceptually different tasks, and it's already a bit of a leaky abstraction on main -- having a packages_skipped attribute on a TestResults instance representing the results of running mypy on one stubs package doesn't really make much sense. I think we should probably separate out the two concepts into two different "result" classes, but this PR feels like it's going in the opposite direction a little bit

[srittau]

We could let test_stdlib() and test_third_party_distribution() just return mypy's return code and files tested, but all other functions are already aggregating results.

[AlexWaygood]

Thanks, this looks great! Just one more optional nit:

[AlexWaygood]

Looks great. Thanks!