Running pytests in parallel

[jeremyleung521]

Issue Number. Is this pull request related to any outstanding issues? If so, list the issue number.

Describe the changes made. A clear and concise description of what the problem is and what you did to fix it. E.g. [...] was happening and I've changed [...] to fix it.
pytest-xdist is a plugin that allows us to run our pytests in parallel. For example, MacOS runners on GitHub Actions have 4 workers, but isn't being taken advantaged of.

Tests are distributed among workers randomly, so there might be more instances of errors when tests are executed out-of-order.

Examples of success: https://github.com/jeremyleung521/westpa/actions/runs/7012321093/job/19111251133
Linux tests that used to take 4 minutes now take 1.5 minutes.

Goals and Outstanding Issues. A clear and concise list of goals (to be) accomplished.

• Speed up GitHub Action tests
• Tests will continue to run even if it fails on a single configuration

Major files changed.

• .github/workflows/test.yaml
• devtools/conda-envs/test_env.yaml
• setup.py

Status.

• New feature (non-breaking change which adds functionality)
• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

Additional context. Add any other context or screenshots about the pull request here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4934c21) 41.89% compared to head (6138859) 41.84%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #363      +/-   ##
===========================================
- Coverage    41.89%   41.84%   -0.05%     
===========================================
  Files          131      131              
  Lines        17011    17011              
===========================================
- Hits          7127     7119       -8     
- Misses        9884     9892       +8     

Flag	Coverage Δ
unittests	41.84% <ø> (-0.05%)	⬇️

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[jeremyleung521]

Results are inconsistent so might have to implement our own scheduler. Will turn this into draft for now.

Some resources:
pytest-dev/pytest-xdist#18
https://stackoverflow.com/questions/59498774/how-to-tell-pytest-xdist-to-run-tests-from-one-folder-sequencially-and-the-rest