TEST: Add test_user_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 user_by_recent_filter_valid().
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2730
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>