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.