software/slapos-testing: fixup don't use custom branch anymore

Fixup of a3e15fc4.
8 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed Cloudooo.UnitTest-Master

00:12:57

passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

00:43:29

failed SlapOS.Eggs.UnitTest-Master.Python2

00:35:42

passed SlapOS.Eggs.UnitTest-Master.Python3

00:26:04

failed SlapOS.Eggs.UnitTest-Master.Python3Next

03:54:07

failed SlapOS.SoftwareReleases.IntegrationTest-Master

10:07:43

passed SlapOS.Theia.ResilienceTest-Master-ERP5

12:11:55

passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

00:45:17