Flaky-test: PendingAckPersistentTest.testPendingAckLowWaterMarkRemoveFirstTxn

[nicoloboschi]

PendingAckPersistentTest.testPendingAckLowWaterMarkRemoveFirstTxn is flaky. It fails sporadically.

example failure

      java.lang.AssertionError: expected [false] but found [true]
at org.testng.Assert.fail(Assert.java:99)
at org.testng.Assert.failNotEquals(Assert.java:1037)
at org.testng.Assert.assertFalse(Assert.java:67)
at org.testng.Assert.assertFalse(Assert.java:77)
at org.apache.pulsar.broker.transaction.pendingack.PendingAckPersistentTest.testPendingAckLowWaterMarkRemoveFirstTxn(PendingAckPersistentTest.java:567)

[nicoloboschi]

I believe it is related to #15530 @liangyepianzhou, can you take a look ?

[liangyepianzhou]

The test has been fixed by #15592

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