replacing '.format' with f-strings in some test files
- [X] contributes to #29547
- [ ] tests added / passed
- [X] passes `black pandas`
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

One format that I left untouched in `pandas/tests/util/test_assert_frame_equal.py`:

```
def test_frame_equal_unicode(df1, df2, msg, by_blocks_fixture, obj_fixture):
    # see gh-20503
    #
    # Test ensures that `tm.assert_frame_equals` raises the right exception
    # when comparing DataFrames containing differing unicode objects.
    msg = msg.format(obj=obj_fixture)
    with pytest.raises(AssertionError, match=msg):
        tm.assert_frame_equal(df1, df2, by_blocks=by_blocks_fixture, obj=obj_fixture)
```

However it looks the the test function isnt being used anywhere:
```
$ grep -rnw . -e "test_frame_equal_unicode"
./pandas/tests/util/test_assert_frame_equal.py:217:def test_frame_equal_unicode(df1, df2, msg, by_blocks_fixture, obj_fixture):
```

Wasn't sure if I should leave as is? or assume person calling the function will format `msg` properly before passing to the test function.