tests: repair --random_activity_priority argument

This stopped working when the signature of Message.__init__ changed in
06aee0c2 (CMFActivity.ActivityTool: Use uid for identity check instead of
oid., 2019-09-17)
6 jobs for fix/random_activity_priority in 0 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-TestRunner1

11:57:13

passed ERP5.PerformanceTest-TestRunner1

02:52:24

failed ERP5.UnitTest-TestRunner1

14:16:26

passed ERP5.CodingStyleTest-TestRunner1

00:36:29

passed ERP5.PerformanceTest-TestRunner1

00:24:48

failed ERP5.UnitTest-TestRunner1

01:12:14