OAK-1679 LdapLoginTestBase#testConcurrentLoginSameGroup fails if conflict handling is enabled

fixed by retrying user synchronization a couple of times.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/branches/1.0@1585380 13f79535-47bb-0310-9956-ffa450edef68