software/erp5/test: tolerate errors in TestPublishedURLIsReachableMixin

Sometimes during the first request to zope mariadb is not started yet and we
have an Site Error. In this case retry several times until we have a sucessful
response.
2 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

10:10:55

passed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

05:08:10