ERP5TypeFunctionalTestCase: Improve initial login realism.

This currently only works because CookieCrumbler reacts on *all* urls which
contain the magic login & password fields. But the POST request body lacks
any value from submit fields, which bypasses the "logged-in" scripts, which
is harmless with current code but lack realism.
6 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

00:45:27

passed ERP5.PerformanceTest-Master

00:26:24

failed ERP5.UnitTest-Master.Medusa

01:16:50

passed SlapOS.Eggs.UnitTest-Master.Python2

00:11:22

passed SlapOS.Eggs.UnitTest-Master.Python3

00:14:06

passed SlapOS.Eggs.UnitTest-Master.Python2

00:13:15