Makes unit tests runner more compatible between operating systems - #693

It is a good idea to run unit tests as an external process and not by directly calling run_unit_tests method. In the current approach unit tests are always executed by using the Python interpreter that runs run_tests.py script.

The implementation to run tests using sys.executable doesn't make much sense, though, because it ends up using the exact same interpreter that is used for running run_tests.py. There's also a problem that on Jython sys.executable may not always be set correctly.

A good way to accomplish the original goal would be using the given interpreter to run the unit tests. This should be done after the larger acceptance test cleanup done in PR #710, though. Or perhaps I could add this enhancement into that PR as well.

Will need rewrite after merging #716.
(Will use --interpreter value)

Closing in favor of #720