Mac/OS X compatibility & unit tests

[rugk]

I tried to run the tests in OS X via Travis, but this failed horribly. See this build for when this is happening.

I also see that the main borgcron.sh script fails (it e.g. cannot do locking) and thus it is very likely incompatible.
If anyone wants to fix that, make the script compatible with OS X and fix the unit tests, feel free to do so. PRs are appreciated.

You would also have to fix things like these strange distorted character display, which comes from here and is correctly displayed only when the variable is accessed another time. (?! 😯)