Separate tests specific to tslibs modules
Going through the tests in tests/scalars, tests/indexes/datetimes, tests/indexes/timedeltas, tests/indexes/period, there is a lot of duplication and a lot of thematically inappropriate placement.  This is a natural result of tests being added over the years.

One result of this, though, is that its easy to miss certain corner cases (#17991, #7996).

There are two things I'd like to do here.  First is gather tests specific to `tslibs` modules so they can be self-contained.  This PR starts that for `tslibs.parsing`.

Second is to go through tests.indexes centralize arithmetic tests, deduplicate where needed (#18026).  This is a big task without an immediate payoff, so I want to get the go-ahead before jumping in.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
