[red-knot] Add mdtests for `global` statement (#17563)
## Summary
This is a first step toward `global` support in red-knot (#15385). I
went through all the matches for `global` in the `mypy/test-data`
directory, but I didn't find anything too interesting that wasn't
already covered by @carljm's suggestions on Discord. I still pulled in a
couple of cases for a little extra variety. I also included a section
from the
[PLE0118](https://docs.astral.sh/ruff/rules/load-before-global-declaration/)
tests in ruff that will become syntax errors once #17463 is merged and
we handle `global` statements.
I don't think I figured out how to use `@Todo` properly, so please let
me know if I need to fix that. I hope this is a good start to the test
suite otherwise.
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Co-authored-by: Carl Meyer <carl@astral.sh>
