CI: Update and fix

[jarzec]

• Update the regression-tests/run-tests.sh
  • Unify the checks using git diff.
  • Add a command line argument -l for setting a label of the test run to be use in the name of the patch file (if it is created).
  • Update the extension of the generated patch files to be .patch which seems to be the accepted convention.
• Update the .github/workflows/regression-tests.yml workflow file.
  • Update the checkout action to avoid GitHub action deprecation warnings.
  • Update the name of the eventual patch artefact file to include the OS identification.
• Update MacOS tests after 29a1f43687192212bd02c2b1da0b2731d7aa2fe4.

[hsutter]

Thanks!