Unwrap most unittest test cases to bare functions

[akx]

Split out from #1240.

This doesn't touch TestCase classes using setup/teardown, just moves them into separate files.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.05%. Comparing base (c5fb390) to head (12f1b70).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1241   +/-   ##
=======================================
  Coverage   92.05%   92.05%           
=======================================
  Files          27       27           
  Lines        4680     4680           
=======================================
  Hits         4308     4308           
  Misses        372      372           

Flag	Coverage Δ
macos-14-3.10	91.08% <ø> (ø)
macos-14-3.11	91.02% <ø> (ø)
macos-14-3.12	91.23% <ø> (ø)
macos-14-3.13	91.23% <ø> (ø)
macos-14-3.14	91.21% <ø> (ø)
macos-14-3.8	90.95% <ø> (ø)
macos-14-3.9	91.01% <ø> (ø)
macos-14-pypy3.10	91.08% <ø> (ø)
ubuntu-24.04-3.10	91.11% <ø> (ø)
ubuntu-24.04-3.11	91.04% <ø> (ø)
ubuntu-24.04-3.12	91.26% <ø> (ø)
ubuntu-24.04-3.13	91.26% <ø> (ø)
ubuntu-24.04-3.14	91.23% <ø> (ø)
ubuntu-24.04-3.8	90.97% <ø> (ø)
ubuntu-24.04-3.9	91.04% <ø> (ø)
ubuntu-24.04-pypy3.10	91.11% <ø> (ø)
windows-2022-3.10	91.10% <ø> (ø)
windows-2022-3.11	91.03% <ø> (ø)
windows-2022-3.12	91.25% <ø> (ø)
windows-2022-3.13	91.25% <ø> (ø)
windows-2022-3.14	91.22% <ø> (ø)
windows-2022-3.8	91.07% <ø> (ø)
windows-2022-3.9	91.03% <ø> (ø)
windows-2022-pypy3.10	91.10% <ø> (ø)

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[akx]

Merging since Codecov is saying "Coverage not affected when comparing c5fb390...12f1b70" (c5fb390 ... 12f1b70), IOW this is a clean refactor.