Add accessibility test on paged.js results

When exporting PDF, the content of the page is updated by paged.js . The result of this process is not checked with accessibility tests.

In order to make sure we provide the best HTML we can to browser exporters, we should check the output of paged.js against our accessibility validator.

Anything past this point is not of our responsibility. Implementing this (with the violations it finds) would mean we can close XWIKI-19144 and XWIKI-19560 . Any additional error would come from the browser exporters.