[ticket/63460] Improve checks for unstable versions

Patch level releases shouldn't e treated as unstable version. In
order to allow proper testing the class variable modx_object of
class mpv_tests_modx had to be changed to be publically accessible.
Additionally, PHP's version_compare() function is now used for
checking for incorrect version.
[ticket/63460] Add tests for mpv_tests_modx::test_version()