Factor out more mdtest helpers

[ntBre]

Summary

This is a follow-up to #24616 to share a bit more infrastructure between ty_test and the ruff_test crate introduced in #24617. In particular, the primary run function has moved into mdtest, along with the attempt_test, check_panic and snapshot_diagnostics helpers.

You should be able to review commit-by-commit, but the first commit is probably the most important to look at since it moves the tracing setup and inconsistency checks from ty_test::run into ty_test::run_test. I think that narrows the span of the tracing logs a bit, but the important section should still be covered.

The other commits should be pretty straightforward and reviewable as one diff.

Test Plan

Existing tests, as well as rebasing #24617 onto this branch.

[astral-sh-bot]

Typing conformance results

No changes detected ✅

Current numbers

The percentage of diagnostics emitted that were expected errors held steady at 87.94%. The percentage of expected errors that received a diagnostic held steady at 83.36%. The number of fully passing files held steady at 79/133.

[astral-sh-bot]

Memory usage report

Memory usage unchanged ✅

[astral-sh-bot]

ecosystem-analyzer results

No diagnostic changes detected ✅

Flaky changes detected. This PR summary excludes flaky changes; see the HTML report for details.

Full report with detailed diff (timing results)

[astral-sh-bot]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.