MAINT,TST: Refactor F2PY testsuite

[HaoZeke]

• Closes Uniform test style for f2py #19922.
• Requires TST,STY: Clean up F2PY tests for pathlib.Path #20479.
  • Not only are some helpers added there, but also the formatting changes to appease flake8 are in that PR
  • Once it is merged this becomes far simpler to review (one commit)

Essentially this makes better usage of the util.py helpers. It will also make testing more transparent and easier to extend since now one can directly write nice Fortran formatted tests instead of massaging lines in python.

Additionally, one point which came up was the speed. AFAIK the time taken to run the tests has decreased, and not increased.

# On main
python runtests.py -m full -s f2py
# 670 passed, 6 skipped in 66.84s (0:01:06)
# On this branch
python runtests.py -m full -s f2py
# 670 passed, 6 skipped in 60.85s (0:01:00)

Similar behavior in terms of time taken was observed in the skeletal test repository.

Other changes:

• Almost no dependencies left on numpy.testing other than IS_PYPY

[pearu]

LGTM!

[melissawm]

This looks good - thanks @HaoZeke