Add precompile .mo test for ploneconf.core package in backend

[fredvd]

No description provided.

[erral]

My comment is not directly related with this change but can't we configure Zope to compile the po files on runtime?

We may need this because all addons available at pypi are not released with mo files, those released using zest.releaser and zest.pocompile will, but everyone don't use them.

[ericof]

@erral We should install packages and compile the mo files in the builder stage and set the compilation of mo files to false during start-up time.