po2moz: handle files with fullstop in filename

E.g. services.device.properties.po - old logic got confused by first
fullstop when extracting the extension properties.po.  So now we find
from the right.
Tests: unhide some tests

Some tests have been hidden because of duplicate method names.  Thanks
Codacy
https://www.codacy.com/public/dwayne/translate/file/169879100/source?bid=1990609#l455