ipatests: fix / permissions for test_nested_group_members
    
    The test test_nested_group_members is performing a ssh login
    with a private key and this command may fail if the root directory
    does not have the right permissions on the ssh server
    (see https://access.redhat.com/solutions/6798261)
    
    Ensure that / has 755 before launching the test.
    
    Fixes: https://pagure.io/freeipa/issue/9615
    Signed-off-by: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Francisco Trivino <ftrivino@redhat.com>