runtime: adjust expected error threshold in TestSelectFairness
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 <387225cd66f97dc812a00599dbc5b6544a73295c@develer.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>

CAFxX/go,akutz/go,cloudflare/go,cloudflare/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go,golang/go,golang/go,akutz/go,akutz/go,nawawi/go,akutz/go,golang/go,akutz/go,nawawi/go,nawawi/go,cloudflare/go,CAFxX/go,CAFxX/go,golang/go,cloudflare/go,akutz/go,golang/go,akutz/go,cloudflare/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,golang/go,akutz/go,cloudflare/go