Cleanup empty tests, add --cores argument

[WardBrian]

Submission Checklist

• Run unit tests
• Declare copyright holder and open-source license: see below

Summary

This does some mypy cleanup following #461. It also closes #467 and closes #466

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):
Simons Foundation

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)

[codecov-commenter]

Codecov Report

Merging #470 (d0b18bc) into develop (832a42d) will decrease coverage by 0.38%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #470      +/-   ##
===========================================
- Coverage    77.87%   77.48%   -0.39%     
===========================================
  Files           30       20      -10     
  Lines         9174     6112    -3062     
===========================================
- Hits          7144     4736    -2408     
+ Misses        2030     1376     -654     

Impacted Files	Coverage Δ
cmdstanpy/cmdstanpy/utils.py	69.94% <0.00%> (-0.58%)	⬇️
cmdstanpy/cmdstanpy/install_cmdstan.py	32.15% <0.00%> (-0.13%)	⬇️
...k/cmdstanpy/cmdstanpy/cmdstanpy/install_cmdstan.py	32.15% <0.00%> (-0.13%)	⬇️
...runner/work/cmdstanpy/cmdstanpy/cmdstanpy/utils.py	70.52% <0.00%> (ø)
a/cmdstanpy/cmdstanpy/cmdstanpy/__init__.py
a/cmdstanpy/cmdstanpy/cmdstanpy/utils.py
a/cmdstanpy/cmdstanpy/cmdstanpy/model.py
a/cmdstanpy/cmdstanpy/cmdstanpy/progress.py
a/cmdstanpy/cmdstanpy/cmdstanpy/_version.py
a/cmdstanpy/cmdstanpy/cmdstanpy/cmdstan_args.py
... and 8 more

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[mitzimorris]

LGTM! cores arg is much needed, ditto cleanup