XWIKI-6882 : Improvements on user changing password. Get rid of groovy.

I added a check password method in the User API, so I don't need Groovy anymore. I hope this would fix failing functional tests.
By the way, note that I had to change the default password of the test users ("test" was too short for being valid), so I guess we have to delete manually the users used for these tests ? (unless Jenkins doesn't reuse the previous test user and re-create them every time we run the tests ?)