CLN: res/exp and GH references in frame tests
EDIT2: reverted import-related clean-up due to https://github.com/pandas-dev/pandas/pull/22730#issuecomment-423844217

EDIT: after review by @WillAyd, the fixturization was split out to #22735, #22736 and #22738. Now contains only the following clean-ups, also for #22733:
* Replace instances of pd. with direct imports (from review https://github.com/pandas-dev/pandas/pull/21899#discussion_r202524353):
  > don’t import pd, directly import instead
* Replace direct use of `assert_..._equal` with the `tm.assert_..._equal`
    (from review https://github.com/pandas-dev/pandas/pull/21899#discussion_r202524374):
  > use tm; don’t import assert_series_equal
* Clean up usage of `exp/res` (from review https://github.com/pandas-dev/pandas/pull/22733#discussion_r218306730):
  > Here you can use `result` and `expected` in spite of the existing code using `exp`
* Unify references to github issues

Original post below. [END EDIT]

- [x] 3 modules of #22471
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

I split the commits per module, and (where possible) into fixturizing and cleaning up the imports. Other notes:
* For `test_apply`, I added an another often-used fixture
* In translating the quasi-fixtures from `TestData` to `conftest` in #22236, I sorted the dtypes for the columns of `mixed_float_frame` and `mixed_int_frame`, which turns out to have been a mistake. This is reverted here to be a true translation of the attribute of `TestData`. Otherwise, tests in the newly fixturized `test_arithmetic.py` would fail.

En passant, I also unified GH issue references.
