test: use ParallelTestRunner.jl

[icweaver]

Use ParallelTestRunner.jl for subpackage tests. Tests are refactored into their own files to leverage parallelization. Below are the relevant speedups in CI:

subpackage	main	PR	main / PR
AstrodynamicalCalculations.jl	1m 17s	48s	1.6x
AstrodynamicalModels.jl	18m 6s	10m 26s	1.7x
AstrodynamicalSolvers.jl	19m 23s	10m 44s	1.8x

Note: Times in main are from #280 to make comparisons more apples-to-apples. Can update once merged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.43%. Comparing base (e44419b) to head (031a1ed).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #277   +/-   ##
=======================================
  Coverage   56.43%   56.43%           
=======================================
  Files          13       13           
  Lines        1212     1212           
=======================================
  Hits          684      684           
  Misses        528      528           

Flag	Coverage Δ
AstrodynamicalCalculations.jl	51.09% <100.00%> (ø)
AstrodynamicalModels.jl	67.97% <ø> (ø)
AstrodynamicalSolvers.jl	64.62% <ø> (ø)
GeneralAstrodynamics.jl	0.00% <ø> (ø)

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[icweaver]

I'm moving the non-ParallelTestRunner bits of this PR over to #280. Will update once that's completed

[icweaver]

Ok, I think things are looking ok here, and with a decent speedup. I tried updating the PR title and top-level comment to follow the conventional commits style. Happy to make any changes!