testutils: fix for secure base url in test client
testutils: fix for secure base url in test client

* Uses `CFG_SITE_SECURE_URL` in `login` and `logout` methods of Flask
  InvenioTestCase.

Signed-off-by: Jiri Kuncar <2bdffcb416fed3002465740d19a053a39918cbe5@cern.ch>

inveniosoftware/invenio,tiborsimko/invenio,tiborsimko/invenio,inveniosoftware/invenio