util-tests: Initialize boolean variable to default value
    
    The boolean variable found_nss could be used uninitialized
    in test test_known_service if service "nss" would not be found.
    We would catch it with valgind.
    
    Reviewed-by: Pavel Reichl <preichl@redhat.com>
    (cherry picked from commit 2f84032c28ac414b77a5c1ad470eb69ed6c6a1b4)