[red-knot] mdtest usability improvements for reveal_type (#13709)
## Summary
Fixes #13708.
Silence `undefined-reveal` diagnostic on any line including a `#
revealed:` assertion.
Add more context to un-silenced `undefined-reveal` diagnostics in mdtest
test failures. This doesn't make the failure output less verbose, but it
hopefully clarifies the right fix for an `undefined-reveal` in mdtest,
while still making it clear what red-knot's normal diagnostic for this
looks like.
## Test Plan
Added and updated tests.
