test: remove time-based checking

[lance6716]

close #988

check a counter to verify cache really takes effect

PR Overview

This PR removes time-based checks in cache tests and replaces them with a counter-based mechanism to verify that caching is effective.

• Removed timing-based measurements from TestCachingSha2Cache and TestCachingSha2CacheTLS.
• Introduced an atomic counter (getCredCallCnt) in RemoteThrottleProvider to confirm that GetCredential is called only once.
• Updated tests to assert the new counter value after establishing connections.

Reviewed Changes

File	Description
server/caching_sha2_cache_test.go	Replaced delay-based timing checks with an atomic counter verification mechanism.

Copilot reviewed 1 out of 1 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

server/caching_sha2_cache_test.go:63

• [nitpick] Consider renaming 'getCredCallCnt' to 'getCredCallCount' for improved clarity.

getCredCallCnt atomic.Int64

[lance6716]

@dveeden PTAL