Fix failing TestSnapshotPersist when not on H2/VCS

TestSnapshotPersist.testSnapshot and TestSnapshotPersist.testPersist where recently observed failing against MongoDB on:

• QA: https://qa.nuxeo.org/jenkins/view/MultiDB/job/master/job/nuxeo-master-fullbuild-mongodb-4.0/464/#showFailuresLink
• Jenkins X:
  • https://jenkins.platform.dev.nuxeo.com/job/nuxeo/job/nuxeo/job/master/398/ 
  • https://jenkins.platform.dev.nuxeo.com/job/nuxeo/job/nuxeo/view/change-requests/job/PR-3891/2/

Yet, it also seems systematic when run locally:

• Against H2:

  mvn -nsu -pl modules/platform/nuxeo-apidoc-server/nuxeo-apidoc-repo test -Dtest=TestSnapshotPersist
  

• Against MongoDB (required having MongoBD installed):

  mvn -nsu -pl modules/platform/nuxeo-apidoc-server/nuxeo-apidoc-repo test -Dtest=TestSnapshotPersist -Dnuxeo.test.core=mongodb
  

TestSnapshotPersist.testSnapshot stack trace attached (similar for testPersist).