importlib.metadata test fixtures should prefer test.support fixtures

[jaraco]

In #116131, I learned that there are some fixtures in importlib.metadata that now appear to have substantial overlap with those in support.os_helper. There was a time when the backport was the dominant implementation and the fixtures needed to stand alone, but as we move to a more stable implementation in the stdlib, the tests should rely on fixtures from the stdlib wherever possible.

Linked PRs

• gh-120801: Refactor importlib.metadata fixtures. #120803