pynfs: fix reporting of ignored tests

currently, ignored tests reported as failure, which
makes fixing them quite hard.

Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
st_destroy_session: remove invalid testcase

we cant expect NFS4ERR_NOT_ONLY_OP if spec
enforces NFS4ERR_OP_NOT_IN_SESSION

Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>