runtime: adjust expected error threshold in TestSelectFairness

Make it a bit more relaxed on the expected fairness, as fastrand()
isn't a truly perfect random number generator.

Fixes #34808

Change-Id: Ib55b2bbe3c1bf63fb4f446fd1291eb1236efc33b
Reviewed-on: https://go-review.googlesource.com/c/go/+/200857


Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>