testutils: login and logout in InvenioTestCase
testutils: login and logout in InvenioTestCase

* Moves methods `login` and `logout` to `InvenioTestCase`.

* One can use `self.login('romeo', 'r123omeo')` inside tests
  instead of manually filling and posting the form.

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

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