Centralize METADATA.toml parsing in the test suite

[AlexWaygood]

We have various bits of logic scattered across the test suite to parse METADATA.toml files. This is a bug magnet: see e.g. #9532 (comment). (Failing CI run here: https://github.com/python/typeshed/actions/runs/3918898414/jobs/6699564673.) By centralizing the logic all in one place, we reduce the risk of bugs in our tests, and also improve type safety.

This PR centralizes METADATA.toml-parsing logic into a new file, tests/parse_metadata.py.

[Avasam]

At this point, this feels redundant since other tests will fail anyway. 😅
Unless you wanna use it as a smoke test.

[AlexWaygood]

I get what you mean, but the other tests will assert the correctness of the METADATA.toml files "by accident". I feel like it's useful to have a test that asserts the correctness of the METADATA.toml files "on purpose"? That way we still have this guaranteed to be tested in CI, even if some other tests no longer need to get stuff from METADATA.toml files in the future, for whatever reason.

[Avasam]

Yeah that's what I meant by a smoke test (an easy test that says, if this fails, everything else will fail anyway, so it's easy to find the source of the error). Just wanted to know if it was on purpose :)

[Avasam]

What about the [mypy-tests] sections?
https://github.com/python/typeshed/pull/9534/files#diff-b42eb36bcaa2d1a163ed1c202e81cffd058915ca695b660d49c636b3a5b56857R199-R216
https://github.com/python/typeshed/blob/main/tests/mypy_test.py#L200
(which doesn't seem to be currently used anywhere?)

[AlexWaygood]

What about the [mypy-tests] sections? https://github.com/python/typeshed/pull/9534/files#diff-b42eb36bcaa2d1a163ed1c202e81cffd058915ca695b660d49c636b3a5b56857R199-R216 (which doesn't seem to be currently used anywhere?)

I believe that if somebody did try to add a per-stub config section for mypy, check_consistent.py would already complain about it. And, as you say, it's not currently used anywhere. I'd prefer not to touch it for this PR.