software/slapos-sr-testing: reorganise py3 tests

html5as-base, grafana and metabase are python3 software releases
and we don't need to run these tests twice for python2 and python3.
2 jobs for feat/mroonga-up
Status Job ID Name Coverage
  External
canceled SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP-Python2

canceled SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP-Python3