Fix `Caching_SendFileWithFullContentLength_Cached()`

[dougbu]

Error Message

Assert.Equal() Failure
↓ (pos 0)
Expected: 1
Actual:   2
↑ (pos 0)

Stacktrace

at Microsoft.AspNetCore.Server.HttpSys.FunctionalTests.ResponseCachingTests.Caching_SendFileWithFullContentLength_Cached() in /_/src/Servers/HttpSys/test/FunctionalTests/ResponseCachingTests.cs:line 401
--- End of stack trace from previous location ---

Build

https://dev.azure.com/dnceng/public/_build/results?buildId=1141411

Notes

Note the changes in #32508 (the PR validated in that build) are incredibly unlikely to cause unreliability in that test. At least anecdotally (per @Tratcher), the test is not reliable. That makes sense when other tests are contending for HTTP connections or otherwise slowing down when the test gets back to issuing its second GetFileAsync(...) call. May need more than a bump to the max-age=10 in the Cache-Control header.

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