Simplify test for negative xerr/yerr.

[anntzer]

1. Move the test into x/y loop, to write it just once.
2. Change the test to directly also work for timedelta, rather than
   having to manually test it. (In practice, the relevant question is
   whether the "low" end of the errorbar is indeed below the "high" end
   of the errorbar.)

PR Summary

PR Checklist

Tests and Styling

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• New features are documented, with examples if plot related.
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).
• Documentation is sphinx and numpydoc compliant (the docs should build without error).