TESTS: Integration test for idle timeout
    
    The secrets responder test was chosen even though the bug was in the generic
    responder code b/c it runs a single responder process, so it's trivial to
    read the PID of the responder under test.
    
    Changes subprocess.call() for os.execv() so that the setup function can
    return the secret responder PID right away.
    
    The client timeout in the test has to be at least 10 seconds because
    internally, the responders don't allow a shorter timeout.
    
    Regression test for https://pagure.io/SSSD/sssd/issue/3448
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>