[Misc] Fix broken plugin caused by the upgrade of the Maven Plugin Plugin version in the top-level POM (see XCOMMONS-964: Convert XAR plugin to use Maven Plugin annotations). I've tried to update the plugin without success so far.

XWIKI-13292: Provide a test module to use as dependency for the module manipulating oldcore test tools

[Misc] try to fix stackoverflow erros with Jing Relax NG on CI by upgrading the version of the NU Validator

[misc] Avoid downloading already embedded dependencies when depending on xwiki-platform-tool-jetty-resources

* Remove pergmen

This reverts commit 42820362.

* REL info and asserts changes as per guidance.

* More consistent prefix.

[Misc] Since we use a third-party connection pool (DBCP by default) we don't need to set Hibernat's internal pool configuration (they're not used). See https://docs.jboss.org/hibernate/orm/3.3/reference/en-US/html/session-configuration.html#configuration-hibernatejdbc for example.

+ Move functional tests to platform

* Make minimal XWiki runtime smaller (that was my original goal for using Servlet 3.0 :))

* Fixed artifact ids to comply with rule

XWIKI-12884: No way to specify a webjars URL for a resource located in another wiki
XWIKI-12878: Webjar installed by EM on a subwiki could not be loaded through the webjar action