[fix][test] Fix flaky test TopicReaderTest.testMultiReaderIsAbleToSeekWithTimeOnMiddleOfTopic

[nicoloboschi]

Motivation

Sometimes this test fails with

java.lang.AssertionError: Received duplicate message msg num 5 expected [true] but found [false]
	at org.testng.Assert.fail(Assert.java:99)
	at org.testng.Assert.failNotEquals(Assert.java:1037)
	at org.testng.Assert.assertTrue(Assert.java:45)
	at org.apache.pulsar.client.api.TopicReaderTest.testMultiReaderIsAbleToSeekWithTimeOnMiddleOfTopic(TopicReaderTest.java:1404)

This is because while the reader tries to seek to a fixed position, the dispatcher have already sent messages starting from the Earliest position.
Even if the consumer on the client side empties the incoming messages after the cursor reset, the messages may already be on the client side and not enqued yet. I think this is a little defeat of the seek feature

Modifications

• Test seek() resetting the cursor before the current position (Latest) and improved the test checks

• doc-not-needed

[mattisonchao]

I think it is not the test problem. maybe we can try to fix the seek operation.

FYI: #16757 (comment)

[nicoloboschi]

OK great, didn't notice that pull, will close this pull