Problem: `vim._with()` has many different use cases which are not
covered with tests.
Solution: cover with tests. Some (many) test cases are intentionally
marked as "pending" because they cover cases which don't work as
expected at the moment (and fixing them requires specific knowledge of
C codebase). Use them as a reference for future fixes.
Also some of "can be nested" tests currently might pass only because
the tested context doesn't work.
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