XWIKI-22272: Add support for testing with multiple users/browsers

* Add new UITest configuration, named 'servletEngineNetworkAliases', to specify additional aliases that can be used to access the servlet engine running XWiki
* Add a helper MultiUserTestUtils that can be injected in test methods allowing to:
** create new browser tabs and associate them with a given XWiki alias (you can log in different users in each of these tabs)
** switch between the created tabs, taking care of restoring the proper form token and base URL
** close the created tabs (except the first one)
** access the handle of the first (main) tab

(cherry picked from commit 22b740a5)