Add ability to manually add the XAR Filter module when running functional tests

When writing new functional tests, it's useful to be able to modify wiki pages and XAR-export them to put them in the source tree.

Since we want the packager plugin to build a minimal XWiki instance we don't bundled the wikistream module by default.

However it would be handy to introduce a system property that a developer could user on the command line when executing the func tests to include it. For example: -Dxwiki.debug.xarexport=true