software/erp5/test: fix flaky test_full_backup

When the backup script takes more than one second the file name is not
20500101000000.sql.gz
14 jobs for feat/theia-a2hs
Status Job ID Name Coverage
  External
passed Cloudooo.UnitTest-Master

passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

00:35:51

passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster.Master

06:46:45

passed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Buster

00:28:37

passed SlapOS.Eggs.UnitTest-Master.Python2

passed SlapOS.Eggs.UnitTest-Master.Python3

passed SlapOS.KVM.ResilienceTest-Master

passed SlapOS.SlapRunner.ResilienceTest-Master

00:22:55

failed SlapOS.SoftwareReleases.IntegrationTest-Master

04:30:01

passed SlapOS.Theia.ResilienceTest-Master

passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

00:30:08

passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

passed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Buster

passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster