[fix][test] Split and fix flaky SASL authentication first-stage auth cache tests

[oneby-wang]

Fixes apache#24337

Motivation

SaslAuthenticateTest.testSaslOnlyAuthFirstStage currently verifies two timing-sensitive behaviors in one test: that first-stage SASL authentication leaves inflight contexts in the cache, and that residual contexts are later expired. The exact cache-size assertion can become flaky when entries expire before the assertion is reached.

Modifications

Added two focused tests for the separate behaviors:

• testSaslOnlyAuthFirstStageKeepsInflightContextsBeforeExpiry verifies that first-stage SASL authentication keeps inflight contexts when the expiry timeout is long enough.
• testSaslOnlyAuthFirstStageExpiresResidualContexts verifies that residual first-stage contexts expire and are removed after cleanup.

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