erp5.util: declare a [test] extra for test dependencies

test_suite and tests_require are for the deprecated `setup.py test`
command that we no longer use.
10 jobs for fix/testnode_proctitle in 0 seconds
Status Job ID Name Coverage
  External
failed ERP5.CodingStyleTest-Master

01:17:37

failed ERP5.UnitTest-Master

05:39:00

failed ERP5.UnitTest-Zope2

02:44:33

passed SlapOS.Eggs.UnitTest-Master.Python2

00:15:25

passed SlapOS.Eggs.UnitTest-Master.Python3

00:16:14

passed SlapOS.Eggs.UnitTest-TestRunner1.Python2

passed SlapOS.Eggs.UnitTest-TestRunner1.Python3

passed Wendelin.UnitTest-Master

00:30:44

passed SlapOS.Eggs.UnitTest-Master.Python2

passed SlapOS.Eggs.UnitTest-Master.Python3