TESTS: Add test for responder_idle_timeout
    
    Two new tests have been added in order to test the following scenarios
    of responder_idle_timeout:
    - responder is shutdown after n seconds;
    - responder has its shutdown delayed due to some activity and then is
      shutdown after n seconds;
    
    In order to have the tests added, a new dep has been introduced:
    python-psutil
    
    Keep in mind those newly added tests make our test suite to take a few
    minutes more to finish. As it may be an inconvenience for some
    developers, the tests have been explicitly marked as slow (both by the
    pytest markdown and by having _slow in their names) and can be skipped
    by doing:
    `make intgcheck-run  make intgcheck-run \
     INTGCHECK_PYTEST_ARGS="-k test_secrets.py -m 'not slow'"`
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3633
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>