Fix race condition in EvictionCount_NotOvercounted_WhenEntryAlreadyRemoved test

[cincuranet]

Use TimeSpan.MaxValue for ExpirationScanFrequency to prevent background ScanForExpiredItems tasks from racing with the test's explicit TryGetValue and Compact operations. The race caused under-counting because a background scan could remove an entry from the ConcurrentDictionary but not yet increment _accumulatedEvictions when GetCurrentStatistics was called.

Fixes #126818

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Pull request overview

This PR stabilizes MemoryCacheMetricsTests.EvictionCount_NotOvercounted_WhenEntryAlreadyRemoved by preventing MemoryCache’s background expiration scanning from racing with the test’s explicit expiration-triggering operations, which previously caused intermittent undercounting of eviction statistics.

Changes:

• Set MemoryCacheOptions.ExpirationScanFrequency to TimeSpan.MaxValue in the affected test to effectively disable background ScanForExpiredItems scheduling during the test.

[mrek-msft]

LGTM. There are TimeSpan.Zero in other tests, but these seems correct to me.

[svick]

It's not directly related to this PR, but would it make sense to support Timeout.InfiniteTimeSpan in MemoryCacheOptions.ExpirationScanFrequency? (Though I think it would technically be a breaking change.)

[cincuranet]

@svick I don't think so. The Timeout.InfiniteTimeSpan is related more to System.Threading. And if you do TimeSpan.MaxValue for ExpirationScanFrequency, it is effectively never.