runtime: make select fairness test less picky

runtime: make select fairness test less picky

Allow up to 10 standard deviations from the mean, instead of
~5 that the current test allows.

10 standard deviations allows up to a 4500/5500 split.

Fixes #52465

Change-Id: Icb21c1d31fafbcf4723b75435ba5e98863e812c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/538815
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>