TST: GH30999 Add placeholder messages to pandas/tests/io/test_sql.py and remove test for numexpr < 2.6.8
This is my attempt to finally finish off #30999

In pandas/tests/io/test_sql.py, there is a whole test class skipped. It looks like xref #20536 is supposed to address that, but no one has commented there since March 2018, so I don't think that's going to be fixed any time soon. I noticed that there were other tests in the same module with `match="<insert message here>"` so I decided to put it in the two tests that I can't figure out the correct error message for.

In pandas/tests/computation/test_compat.py there was an if statement that the numexpr library is at least 2.6.8. I tried to set up an environment with a lower version but conda couldn't resolve the dependencies. That test isn't running in the CI (xref #38876) and that test was last touched in a substantive way in 2016. I think that portion of the test is no longer required.

Not sure that I actually addressed these correctly, but I made an attempt so we could have a conversation about it in a more concrete way.

- [x] xref #30999
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
