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>

src/runtime/chan_test.go

@@ -481,12 +481,13 @@ 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 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 > 4.891676/(2*math.Sqrt(trials)) {
+	// See if we're more than 10 sigma away from the expected value.
+	// 10 sigma is a lot, but we're ok with some systematic bias as
+	// long as it isn't too severe.
+	const mean = trials * 0.5
+	const variance = trials * 0.5 * (1 - 0.5)
+	stddev := math.Sqrt(variance)
+	if math.Abs(float64(cnt1-mean)) > 10*stddev {
 		t.Errorf("unfair select: in %d trials, results were %d, %d", trials, cnt1, cnt2)
 	}
 	close(done)