Prevent failures in tearDownServer()

Caused by: org.nuxeo.ecm.automation.OperationException: Failed to invoke operation NuxeoDrive.TearDownIntegrationTests
	at org.nuxeo.ecm.automation.core.impl.InvokableMethod.invoke(InvokableMethod.java:188)
	at org.nuxeo.ecm.automation.core.impl.OperationChainCompiler$OperationMethod.invoke(OperationChainCompiler.java:128)
	at org.nuxeo.ecm.automation.core.impl.OperationChainCompiler$CompiledChainImpl.invoke(OperationChainCompiler.java:196)
	at org.nuxeo.ecm.automation.core.impl.OperationServiceImpl.run(OperationServiceImpl.java:119)
	at org.nuxeo.ecm.automation.core.impl.OperationServiceImpl.run(OperationServiceImpl.java:107)
	at org.nuxeo.ecm.automation.server.jaxrs.OperationResource.execute(OperationResource.java:58)
	at org.nuxeo.ecm.automation.server.jaxrs.ExecutableResource.doPost(ExecutableResource.java:71)
	... 98 more
Caused by: org.nuxeo.ecm.core.api.NuxeoException: Cannot use closed connection handle: org.nuxeo.ecm.core.storage.sql.ra.ConnectionImpl@6908959f
	at org.nuxeo.ecm.core.storage.sql.ra.ConnectionImpl.getSession(ConnectionImpl.java:146)
	at org.nuxeo.ecm.core.storage.sql.ra.ConnectionImpl.getModel(ConnectionImpl.java:168)
	at org.nuxeo.ecm.core.storage.sql.coremodel.SQLSession.idToString(SQLSession.java:155)
	at org.nuxeo.ecm.core.storage.sql.coremodel.SQLDocumentLive.getUUID(SQLDocumentLive.java:134)
	at org.nuxeo.ecm.core.storage.sql.coremodel.SQLDocumentLive.getVersionSeriesId(SQLDocumentLive.java:442)
	at org.nuxeo.ecm.core.storage.sql.coremodel.SQLDocumentLive.getVersionsIds(SQLDocumentLive.java:518)
	at org.nuxeo.ecm.core.versioning.DefaultVersionRemovalPolicy.removeVersions(DefaultVersionRemovalPolicy.java:77)
	at org.nuxeo.ecm.core.api.AbstractSession.removeNotifyOneDoc(AbstractSession.java:1422)
	at org.nuxeo.ecm.core.api.AbstractSession.removeDocument(AbstractSession.java:1399)
	at org.nuxeo.ecm.core.api.AbstractSession.removeDocument(AbstractSession.java:1389)
	at org.nuxeo.drive.operations.test.NuxeoDriveIntegrationTestsHelper.cleanUp(NuxeoDriveIntegrationTestsHelper.java:88)
	at org.nuxeo.drive.operations.test.NuxeoDriveTearDownIntegrationTests.run(NuxeoDriveTearDownIntegrationTests.java:47)
	at sun.reflect.GeneratedMethodAccessor234.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.nuxeo.ecm.automation.core.impl.InvokableMethod.doInvoke(InvokableMethod.java:164)
	at org.nuxeo.ecm.automation.core.impl.InvokableMethod.invoke(InvokableMethod.java:176)