Tweak breaking groups for comprehensions (#6321)
## Summary
Fixes some comprehension formatting by avoiding creating the group for
the comprehension itself (so that if it breaks, all parts break on their
own lines, e.g. the `for` and the `if` clauses).
Closes https://github.com/astral-sh/ruff/issues/6063.
## Test Plan
Bunch of new fixtures.
