recipe/promise_plugin: Fix tests

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Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Master.Python2

01:05:41

passed SlapOS.Eggs.UnitTest-Master.Python3

00:11:11

passed SlapOS.SlapRunner.ResilienceTest-Master

00:21:25

passed SlapOS.Eggs.UnitTest-Master.Python2

00:16:02

passed SlapOS.Eggs.UnitTest-Master.Python3

00:12:48