Fix random in BatchManagerFixture.testBatchConcurrency on Postgres

Run unit tests / Run postgresql unit tests / org.nuxeo.ecm.automation.server.jaxrs.batch.BatchManagerFixture.testBatchConcurrency
Failing for the past 1 build (Since #326 )
Took 1.4 sec.
Error Message

expected:<0> but was:<224>

Stacktrace

java.lang.AssertionError: expected:<0> but was:<224>
	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.ecm.automation.server.jaxrs.batch.BatchManagerFixture.testBatchConcurrency(BatchManagerFixture.java:369)