Add MacOS Tests to GH Actions

[jeremyleung521]

Issue Number. Is this pull request related to any outstanding issues? If so, list the issue number.
#251 #325

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.
With #251 fixing things, I think we should start running tests on MacOS to make sure everything works. It will take a little longer, but at < 15 minutes, it is fine!
(Example of a working workflow: https://github.com/westpa/westpa/actions/runs/5754524605)

Also forgot to update the test_env.yaml with pytest-rerunfailures and remove nose in #325!

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

• More comprehensive testing!

Major files changed.

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

Status.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• 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

Merging #335 (02cefc6) into develop (e022c31) will increase coverage by 0.04%.
Report is 1 commits behind head on develop.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #335      +/-   ##
===========================================
+ Coverage    41.35%   41.39%   +0.04%     
===========================================
  Files          131      131              
  Lines        16967    16967              
===========================================
+ Hits          7016     7024       +8     
+ Misses        9951     9943       -8     

Flag	Coverage Δ
unittests	41.39% <ø> (+0.04%)	⬆️

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see 2 files with indirect coverage changes