tests: add admin login tests

[alejandromumo]

Update: I can't test whether an admin can view an invenio-administration view using a test application. The request gets redirected and then a FileNotFound: manifest.json error is thrown. It seems to me that the test application does not build assets and, therefore, no manifest.json is found.

client_with_login.get('/administration/') # returns a 302 and redirects to '/'

This seems to be a broader issue also mentioned here. And it seems we're testing a similar behaviour and ran into the same issue in invenio-admin.