Fix: Replace fixed delay with Awaitility in SyslogAppenderCustomLayoutTest

[Kcruz28]

This PR fixes a flaky test that was failing because of a race condition.

This pull request addresses intermittent test failures caused by relying on a fixed delay Thread.sleep() to wait for asynchronous operations. Fixed delays are unreliable because the operation may occasionally take longer than the predefined wait time.

This commit replaces the fixed delay with a polling mechanism using the Awaitility library. Awaitility continuously checks for the expected condition within a specified timeout, ensuring the test only proceeds once the desired state is reached. This makes the test more robust and reliable by synchronizing with actual operation completion rather than arbitrary timing.

Fixes: #3006

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