BUG/TST: run and fix all arithmetic tests with+without numexpr
Closes #40462

This PR adds a auto-used fixture for `pandas/tests/arithmetic` that sets the `expressions._MIN_ELEMENTS` to 0, to force taking the numexpr path, even if our test data are small (which would otherwise never excercise the numexpr code path)

Further, it makes some fixes to get the tests passing with this better coverage. 
The fixes are a bit quick-and-dirty, but at least already show what is needed to get things working.
can you merge master > @jbrockmendel comments addressed?

Not yet This is ready for another review now ~(although there are some failures about a wrong error message with the latest commit, I see, but aside from that should already be reviewable)~ this should be fixed, wrong bracket alignment ;) thanks @jorisvandenbossche 