Introduce offline configuration parameter to speed up TestContainers-based testing

Form the command line:
mvn clean install -Dxwiki.test.ui.offline=true

By defaut it's online so that if you have artifacts missing locally and you're running from your IDE, they'll get fetched remotely. However when building from Maven (eg. on the CI) we can rely solely on the local maven repo and this speeds up the test execution.