JCR-3504: ConcurrentImportTest failure

Mark the failing test as a known issue.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/trunk@1440336 13f79535-47bb-0310-9956-ffa450edef68
2.6: Merged revision 1440336 (JCR-3504)

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/branches/2.6@1440342 13f79535-47bb-0310-9956-ffa450edef68