Random failure on TestFileManagerService.testFileImporterDocType

This test failure is random:

http://qa.nuxeo.org/jenkins/job/nuxeo-master-fullbuild-openjdk7/293/testReport/junit/org.nuxeo.ecm.platform.filemanager/TestFileManagerService/testFileImporterDocType/

because the order of tests execution is random a failure happens only when testFileImportersMerge is done before testFileImporterDocType.

There are no harness.undeployTestContrib so the merge test should be updated to not impact other tests.