erp5.util: update DummyTaskDistributor API

so tests can run locally without reporting to an external server

this change follows 0c7a763e
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  External
passed ERP5.CodingStyleTest-Master

03:08:20

passed SlapOS.Eggs.UnitTest-Master.Python2

02:10:11

passed SlapOS.Eggs.UnitTest-Master.Python3

00:54:02

passed SlapOS.Eggs.UnitTest-Master.Python2

02:07:57

passed SlapOS.Eggs.UnitTest-Master.Python3

00:54:35