Allow passing custom command lines as "PARALLEL_TESTS_EXECUTABLE"

[deivid-rodriguez]

This used to work until 7fd7518. I think it was an oversight so this change adds the proper shellspliting so that it keeps working.

Checklist

• Feature branch is up-to-date with master (if not - rebase it).
• Added tests.
• Added an entry to the Changelog if the new
  code introduces user-observable changes.
• Update Readme.md when cli options are changed

[grosser]

thx, makes sense
... only downside would be anyone trying to have an executable with spaces in it, but then they can escape that 🤷

[grosser]

4.7.2

[deivid-rodriguez]

Thank you!