XWIKI-22625: TestUtils#getURL doesn't take the wiki into account (#3615)

  * Improve getBaseBinURL so that it fallbacks on currentWiki if this
    one is provided
  * Add a new getURL API taking into account the wiki name
  * Extract the wiki from the entityReference when using getURL with a
    reference
  * Fix and cover possible NPE
  * Move wikiName argument to the beginning to follow the URL scheme
  * Remove Unstable
  * minor fix

(cherry picked from commit 77c5da6d)