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>

src/runtime/chan_test.go

@@ -480,11 +480,11 @@ func TestSelectFairness(t *testing.T) {
 	// If the select in the goroutine is fair,
 	// cnt1 and cnt2 should be about the same value.
 	// With 10,000 trials, the expected margin of error at
-	// a confidence level of five nines is 4.4172 / (2 * Sqrt(10000)).
+	// a confidence level of six nines is 4.891676 / (2 * Sqrt(10000)).
 	r := float64(cnt1) / trials
 	e := math.Abs(r - 0.5)
 	t.Log(cnt1, cnt2, r, e)
-	if e > 4.4172/(2*math.Sqrt(trials)) {
+	if e > 4.891676/(2*math.Sqrt(trials)) {
 		t.Errorf("unfair select: in %d trials, results were %d, %d", trials, cnt1, cnt2)
 	}
 	close(done)