Fix UniqueBlockingQueueTest.concurrentTest

[secfree]

What changes are proposed in this pull request?

Fix UniqueBlockingQueueTest.concurrentTest

Why are the changes needed?

This test may fail some times like below

[ERROR] alluxio.util.executor.UniqueBlockingQueueTest.concurrentTest  Time elapsed: 0.003 s  <<< FAILURE!
java.lang.AssertionError: expected:<20> but was:<16>
	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 alluxio.util.executor.UniqueBlockingQueueTest.concurrentTest(UniqueBlockingQueueTest.java:54)

Does this PR introduce any user facing changes?

NO