setup: declare test dependences with extras_require

Only setup.py test understands tests_require
8 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Master.Python2

00:23:38

passed SlapOS.Eggs.UnitTest-Master.Python3

00:26:04

failed SlapOS.Eggs.UnitTest-Master.Python2

00:24:15

failed SlapOS.Eggs.UnitTest-Master.Python2

passed SlapOS.Eggs.UnitTest-Master.Python2

00:18:12

failed SlapOS.Eggs.UnitTest-Master.Python3

00:20:24

failed SlapOS.Eggs.UnitTest-Master.Python3

failed SlapOS.Eggs.UnitTest-Master.Python3

00:20:48