Change pysaml2 comment in test-requrements.txt.

    Even though the ``pysaml2`` library is used for Keystone2Keystone
    functionalities, Keystone will complain about missing dependency right
    after federation extension is enabled.
    Commend in the test-requirements.txt should be updated to reflect the
    situation.

Change-Id: Id4e615905574d5ad3d050bd8077b25f8327419ab
Merge "Change pysaml2 comment in test-requrements.txt."