As far as sassc is available in all images... enable it for phpunit runs

[stronk7]

All the moodle images do include the sassc compiler.

So we can enable related phpunit tests by default.

This is about to:

1. Unconditionally add define('PHPUNIT_PATH_TO_SASSC', '/usr/bin/sassc');
2. Cover the existence of the sassc executable with tests.

Ciao :-)