BUILD: Speed up build of some tests
    
    Some tests were built with files require for backend $(sssd_be_SOURCES).
    This automake variable contains 15 files
    which were build every time for each test.
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit 802909e59daa52c734dbe7f8fa13b0ee23e3e576)