DEPR: deprecate pandas.tests

[twoertwein]

• closes TST/TYP: make pandas/tests private #46651
• Tests added and passed if fixing a bug or adding a new feature
• 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):

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"

[pep8speaks]

Hello @twoertwein! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

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

Comment last updated at 2022-07-16 18:27:17 UTC

[mroeschke]

Could be tested by running that python command in a subprocess and inspecting stdout?

[twoertwein]

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.