Fix TestSQLBackendArrayColumns.tes​tParallelArrayUpdate on PostgreSQL

org.nuxeo.ecm.core.storage.sql.TestSQLBackendArrayColumns.testParallelArrayUpdate
Failing for the past 16 builds (Since #1656 )
Took 0.83 sec.
Error Message

[a, x, x, x, x, x, x] expected:<26> but was:<7>

Stacktrace

java.lang.AssertionError: [a, x, x, x, x, x, x] expected:<26> but was:<7>
	at org.junit.Assert.fail(Assert.java:88)
	at org.junit.Assert.failNotEquals(Assert.java:743)
	at org.junit.Assert.assertEquals(Assert.java:118)
	at org.junit.Assert.assertEquals(Assert.java:555)
	at org.nuxeo.ecm.core.storage.sql.TestSQLBackend.testParallelArrayUpdate(TestSQLBackend.java:475)