software/erp5/test: fix flaky test_long_request_interrupted_on_default_family

The default family was also processing activities, so it happened
sometimes that the zope does not respond in time because it's processing
activities. Create a dedicated activity family so that the default zope
is available to serve requests.
Status Job ID Name Coverage
  External
running Cloudooo.UnitTest-Master

23229:34:01

running SlapOS.SlapRunner.ResilienceTest-Master

23229:33:58

failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP-Python2

07:28:39

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

04:56:21

running SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

23229:34:08

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

05:31:02

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