fix: Adjust tests to not depend on how pytest is invoked

[effigies]

python -m pytest triggers changes in argparse behavior (relative to pytest) as of python/cpython#124799. Let's start by running some of our CI jobs with python -m pytest to replicate #1446.

Edit: Test failures in #1446 and #1476 replicated. Patching both.

Closes #1446.
Closes #1476.

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.41%. Comparing base (db76723) to head (d8653ec).
⚠️ Report is 6 commits behind head on maint/5.4.x.

Files with missing lines	Patch %	Lines
nibabel/tests/test_removalschedule.py	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           maint/5.4.x    #1478      +/-   ##
===============================================
- Coverage        95.42%   95.41%   -0.01%     
===============================================
  Files              209      209              
  Lines            29820    29822       +2     
  Branches          4482     4483       +1     
===============================================
  Hits             28455    28455              
- Misses             930      931       +1     
- Partials           435      436       +1     

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