Flaky-test: PulsarOffsetBackingStoreTest.testGetSet

[example failure](https://github.com/apache/pulsar/runs/7714287203?check_suite_focus=true)

```
Error: Tests run: 17, Failures: 1, Errors: 0, Skipped: 15, Time elapsed: 58.95 s <<< FAILURE! - in org.apache.pulsar.io.kafka.connect.PulsarOffsetBackingStoreTest
Error: testGetSet(org.apache.pulsar.io.kafka.connect.PulsarOffsetBackingStoreTest) Time elapsed: 0.236 s <<< FAILURE!
java.lang.AssertionError: expected [9] but found [10]
at org.testng.Assert.fail(Assert.java:99)
at org.testng.Assert.failNotEquals(Assert.java:1037)
at org.testng.Assert.assertEqualsImpl(Assert.java:140)
at org.testng.Assert.assertEquals(Assert.java:122)
at org.testng.Assert.assertEquals(Assert.java:907)
at org.testng.Assert.assertEquals(Assert.java:917)
at org.apache.pulsar.io.kafka.connect.PulsarOffsetBackingStoreTest.testGetSet(PulsarOffsetBackingStoreTest.java:163)
at org.apache.pulsar.io.kafka.connect.PulsarOffsetBackingStoreTest.testGetSet(PulsarOffsetBackingStoreTest.java:126)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:568)
at org.testng.internal.MethodInvocationHelper.invokeMethod(MethodInvocationHelper.java:132)
at org.testng.internal.InvokeMethodRunnable.runOne(InvokeMethodRunnable.java:45)
at org.testng.internal.InvokeMethodRunnable.call(InvokeMethodRunnable.java:73)
at org.testng.internal.InvokeMethodRunnable.call(InvokeMethodRunnable.java:11)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
at java.base/java.lang.Thread.run(Thread.java:833)
```

Start with PulsarOffsetBackingStoreTest.testGetSet, especially the assertion at line 163 and the reported example failure. Investigate why the test sometimes finds 10 instead of the expected 9, then run the test repeatedly to confirm the failure is resolved and the expected result is stable.

Written by the indexing model from the issue text.