Use `sort` fixture in more places
This is a follow-up on [this review comment by jbrockmendel](https://github.com/pandas-dev/pandas/pull/32046#discussion_r382697823).

There's a `sort` fixture in `tests.indexes.interval.test_setops`, which could be used in other places as well. For instance in `test_difference_base` in `pandas/tests/indexes/common.py`.

There are probably many more places where it could be used.

