Add test for duplicate PYTHONPATH in modextrapaths

[Flamefire]

And similar test for sanity_check_paths.

Intended to avoid issues fixed in #13385

Edit: Changed to using regexs to catch hard-coded version numbers instead of %(pyshortver)s.

Example failure using one of the faulty ECs from #13385:

CIRCexplorer-1.1.10-intel-2017b-Python-2.7.14.eb: sanity_pip_check should be enabled
CIRCexplorer-1.1.10-intel-2017b-Python-2.7.14.eb: modextrapaths contains superflous 'lib/python2.7/site-packages' (automatically added by easyblock)
CIRCexplorer-1.1.10-intel-2017b-Python-2.7.14.eb: sanity_check_paths['dirs'] contains superflous ['lib/python2.7/site-packages'] (automatically added by easyblock)

[Flamefire]

@boegel Anything missing here? This is a CI test change only so shouldn't cause any regressions for users (but actually helps avoiding them)