software/gitlab: run git-backup test during software step

To fail early when there is something wrong in the environment. We are
seeing git crashes and incompatibilities with git versions
10 jobs for fix/gitlab-rst in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-TestRunner1.Python2

00:13:54

passed SlapOS.Eggs.UnitTest-TestRunner1.Python3

00:15:20

failed SlapOS.SoftwareReleases.IntegrationTest-TestRunner1

07:01:32

failed SlapOS.SoftwareReleases.IntegrationTest-TestRunner1-DebianNext

35:21:35

passed SlapOS.Eggs.UnitTest-TestRunner1.Python2

00:17:58

passed SlapOS.Eggs.UnitTest-TestRunner1.Python2

00:13:25

passed SlapOS.Eggs.UnitTest-TestRunner1.Python3

00:22:51

passed SlapOS.Eggs.UnitTest-TestRunner1.Python3

00:14:21

failed SlapOS.SoftwareReleases.IntegrationTest-TestRunner1

09:04:29

passed SlapOS.SoftwareReleases.IntegrationTest-TestRunner1

05:00:03