TEST: Add test_users_by_recent_filter_valid
    
    Test users_by_filter_valid() was removed in past. We will add two new
    tests instead of it. Logic of those tests is connected to RECENT
    filter. It returns only records which have been wrote or updated
    after filter was created (or another given time).
    
    users_by_filter_valid() --> user_by_recent_filter_valid()
                                users_by_recent_filter_valid()
    
    The first of new tests, user_by_recent_filter_valid(), counts with
    two users. One is stored before filter request creation and the second
    user is stored after filter request creation. So filter returns only one
    user.
    
    The second of new tests, users_by_recent_filter_valid(), counts with
    three users. One is stored before filter request creation and two
    users are stored after filter request creation. So filter returns two
    users.
    
    This patch adds users_by_recent_filter_valid().
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2730
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit a6a5a08a357d2adbb653b81bacc602ca3543c4c4)