TST: Collect/Use arithmetic test fixtures
Takes over from #22350 (which would have been a rebasing nightmare), addressing AFAICT all unaddressed comments from there.

All remaining fixtures in tests/arithmetic are collected in tests/arithmetic/conftest.py
Fixtures are given more descriptive names and docstrings as requested in #22350.

A handful of tests are altered to use fixtures; in a number of cases their `pytest.mark.parametrize` arguments were already equivalent to one of the fixtures.

A few new broken cases are identified, specifically numeric `Series` and `DataFrame` `__mul__` or `__rdiv__` with a) non-nanosecond `timedelta64` or b) `Tick`.  (I'll open an Issue for these)

Some tests got cleaned up nicely by using `tm.assert_equal` instead of parametrizing with `assert_func`