Add branch for MacOS 10.13 when checking 'date' command

[NightFlyer]

Before submitting a pull request, please make sure the following is done:

• Fork the repository and create your branch from master.
• If you've added code that should be tested, add tests!
• Ensure the test suite passes.
  It does now that the harness correctly processes dates on Mac OS 10.13!
• Format your code with ShellCheck.
• Include a human-readable description of what the pull request is trying to accomplish.
  Fix test harness for MacOS 10.13 so that date tests work correctly.
• Steps for the reviewer(s) on how they can manually QA the changes.
  Run on MacOS 10.13
• Have a fixes #XX reference to the issue that this pull request fixes.
  fixes Test harness doesn't support Mac OS 10.13 #277

[inkarkat]

I've never been involved in the MacOS platform (best I could do was testing on BSD, as I don't have any Apple hardware). This change is making the ugly stuff a bit worse, but if it's needed... The alternative would be to drop those special cases and demand that the user supplies a compatible date wrapper.

[eightyfive]

I did a smaller (and more future prone) fix FYI:
#291