DEPR: deprecate pandas.tests
- [x] closes #46651
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Not sure how to test this. All of the following imports trigger the warning (DeprecationWarning is by default not enabled, which hopefully doesn't spam the CI):
```sh
python -W default -c "from pandas import tests"
python -W default -c "from pandas.tests.frame import common"
python -W default -c "import pandas.tests.frame.common"
```

Hello @twoertwein! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:







There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2022-07-16 18:27:17 UTC > Could be tested by running that python command in a subprocess and inspecting stdout?

I added such a test case but for some reason, it doesn't trigger the warning. I don't know why.