Deflake TestFnCacheSanity (#10250)
Deflake TestFnCacheSanity (#10250)

Increase tolerance on expected reads.

This should prevent failures where due to our approximation, we estimate
a fractional number of reads that exceed our tolerance of 1.

Sample error: Max difference between 10.461059975 and 9 allowed is 1, but difference was 1.4610599749999995

Updates #9492
gravitational/teleport,gravitational/teleport,gravitational/teleport,gravitational/teleport,gravitational/teleport,gravitational/teleport