Fix TestDocumentImport.fourStepsImport, randomly failing against PostgreSQL

Regression
Run unit tests / Run postgresql unit tests / org.nuxeo.importer.stream.tests.importer.TestDocumentImport.fourStepsImport

expected:<1000> but was:<410>
Stacktrace
java.lang.AssertionError: expected:<1000> but was:<410>
	at org.junit.Assert.fail(Assert.java:89)
	at org.junit.Assert.failNotEquals(Assert.java:835)
	at org.junit.Assert.assertEquals(Assert.java:647)
	at org.junit.Assert.assertEquals(Assert.java:633)
	at org.nuxeo.importer.stream.tests.importer.TestDocumentImport.fourStepsImport(TestDocumentImport.java:176)

Caused by: java.sql.BatchUpdateException: Batch entry 0 INSERT INTO "hierarchy" ("id", "parentid", "pos", "name", "isproperty", "primarytype", "mixintypes", "ischeckedin", "baseversionid", "majorversion", "minorversion", "isversion", "isrecord", "retainuntil", "haslegalhold", "isretentionactive", "istrashed", "systemchangetoken", "changetoken") VALUES ('6d9fdde0-b179-4c0c-92b7-f2624164b6cf', '84876328-96e9-4e8e-9c12-4c77da0e3d99', NULL, 'doughs-tann', 'FALSE', 'File', NULL, NULL, NULL, 0, 0, NULL, NULL, NULL, NULL, NULL, NULL, 0, 0) was aborted: ERROR: duplicate key value violates unique constraint "hierarchy_unique_child"
  Detail: Key (parentid, name)=(84876328-96e9-4e8e-9c12-4c77da0e3d99, doughs-tann) already exists.  Call getNextException to see other errors in the batch.
	at org.postgresql.jdbc.BatchResultHandler.handleError(BatchResultHandler.java:153) ~[postgresql-42.2.9.jar:42.2.9]
	at org.postgresql.core.ResultHandlerDelegate.handleError(ResultHandlerDelegate.java:50) ~[postgresql-42.2.9.jar:42.2.9]
	at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:2242) ~[postgresql-42.2.9.jar:42.2.9]
	at org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:508) ~[postgresql-42.2.9.jar:42.2.9]
	at org.postgresql.jdbc.PgStatement.internalExecuteBatch(PgStatement.java:850) ~[postgresql-42.2.9.jar:42.2.9]
	at org.postgresql.jdbc.PgStatement.executeBatch(PgStatement.java:873) ~[postgresql-42.2.9.jar:42.2.9]
	at org.postgresql.jdbc.PgPreparedStatement.executeBatch(PgPreparedStatement.java:1562) ~[postgresql-42.2.9.jar:42.2.9]
	at org.tranql.connector.jdbc.StatementHandle.executeBatch(StatementHandle.java:155) ~[tranql-connector-1.8.jar:1.8]
	at org.nuxeo.ecm.core.storage.sql.jdbc.JDBCRowMapper.insertSimpleRows(JDBCRowMapper.java:535) ~[classes/:?]
	... 30 more
Caused by: org.postgresql.util.PSQLException: ERROR: duplicate key value violates unique constraint "hierarchy_unique_child"
  Detail: Key (parentid, name)=(84876328-96e9-4e8e-9c12-4c77da0e3d99, doughs-tann) already exists.