ipautil.run: add test for runas parameter
    
    Add a test for ipautil.run() method called with runas parameter.
    The test is using ipautil.run() to execute /usr/bin/id and
    checks that the uid/gid are consistent with the runas parameter.
    
    Note that the test needs to be launched by the root user
    (non-privileged user may not have the rights to execute ipautil.run()
    with runas parameter).
    
    Related to: https://pagure.io/freeipa/issue/7681
    
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>