Fix flaky WebFluxSse integration test

[chemicL]

The WebFluxSseIntegrationTests.testLoggingNotification test is flaky and can fail with:

Expected size: 3 but was: 2 in:
[LoggingMessageNotification[level=NOTICE, logger=test-logger, data=Notice message],
    LoggingMessageNotification[level=ERROR, logger=test-logger, data=Another error message]]

When running this test in the IDE with 100 times (with 2 test params it means 200 runs) repeat mode, it can take 25s in the optimistic variant on my machine and occasionally fails for the JDK HttpClient variant.

With this change, repeating 100 times finishes in 4s. It essentially avoids using awaitility and instead relies on a CountDownLatch to coordinate completion. The flakiness comes from the fact that the results are aggregated in an ArrayList, which can fail to publish the results coming from asynchronous sources. The replacement used is a CopyOnWriteArrayList.