kernel/synflood: reduce sampling time and increase duration
    
    On fast systems with a lot of cores the 10s synflood run produces too
    large perf.data (> 1 GiB), so reduce it to 5s and also increase the
    test's time limit to 1h to make sure that it has enough time to process
    such a large file.
    
    Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>