Flaky testResourceGroupManager tests

[raghavsethi]

This causes ~30% of Travis runs to fail.

'''
Tests run: 1696, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 2,623.875 sec <<< FAILURE!
testResourceGroupManager(com.facebook.presto.execution.TestQueues) Time elapsed: 4.925 sec <<< FAILURE!
java.lang.AssertionError: expected [2] but found [1]
at org.testng.Assert.fail(Assert.java:94)
at org.testng.Assert.failNotEquals(Assert.java:494)
at org.testng.Assert.assertEquals(Assert.java:123)
at org.testng.Assert.assertEquals(Assert.java:265)
at org.testng.Assert.assertEquals(Assert.java:275)
at com.facebook.presto.execution.TestQueues.testBasic(TestQueues.java:95)
at com.facebook.presto.execution.TestQueues.testResourceGroupManager(TestQueues.java:52)
'''

[cberner]

Should be fixed by #6504