test: workaround a flaky source test

[xxchan]

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What's changed and what's your intention?

This test becomes flaky. e.g., https://buildkite.com/risingwavelabs/pull-request/builds/63945

Probable reason: After #19467, CREATE SOURCE returns earlier than before (previously, it will wait for source reader created, i.e., connection established). Now it doesn't wait for that.
See also #19681 (comment)

Checklist

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[xxchan]

still failed...

[StrikeW]

Previously I increased sleep to 6s and also add flush to release-2.1, but the main-cron for v2.1 still failed in release test. Rerun again it can pass.
Actually I didn't sure where the race condition exist, would appreciate any insight from you.

[xxchan]

sleep6s + flush can still fail

[xxchan]

In this run. it takes ~1.2s to receive msg

#first log
2024-12-10T09:10:08.602685725Z DEBUG risingwave_connector::source::reader::reader: spawning connector split reader for multiple splits [Kafka(KafkaSplit { topic: "test-topic-19563", partition: 1, start_offset: Some(-1), stop_offset: None })]
...
#last log
2024-12-10T09:10:09.784461512Z  INFO risingwave_connector::source::kafka::source::reader: [KAFKA] MESSAGE RECEIVED: [Ok(Message { ptr: 0x7f00b2811f78 }), Ok(Message { ptr: 0x7f00b28120b8 }), Ok(Message { ptr: 0x7f00b28121f8 }), Ok(Message { ptr: 0x7f00b2812338 }), Ok(Message { ptr: 0x7f00b2812478 })] self.splits=[KafkaSplit { topic: "test-topic-19563", partition: 3, start_offset: Some(-1), stop_offset: None }, KafkaSplit { topic: "test-topic-19563", partition: 4, start_offset: Some(-1), stop_offset: None }]

[xxchan]

Cannot trigger the failure any more after adding logs..

[xxchan]

Great, this time it failed

it takes 1min14s. And no message consumed at all.

2024-12-10T10:10:40.638303204Z
...
2024-12-10T10:11:54.516867539Z

[xxchan]

Strange. We can see the state table has latest offsets, but no data emited.

[xxchan]

See here for the analysis.
#19681 (comment)

Let's make it not flaky first to avoid troubling others.