math/rand: use t.Helper in tests

Change-Id: Iece39e6412c0f6c63f563eed1621b8cca02de835
Reviewed-on: https://go-review.googlesource.com/51890
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Avelino <t@avelino.xxx>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/math/rand/rand_test.go

@@ -74,6 +74,7 @@ func getStatsResults(samples []float64) *statsResults {
 }
 
 func checkSampleDistribution(t *testing.T, samples []float64, expected *statsResults) {
+	t.Helper()
 	actual := getStatsResults(samples)
 	err := actual.checkSimilarDistribution(expected)
 	if err != nil {
@@ -82,6 +83,7 @@ func checkSampleDistribution(t *testing.T, samples []float64, expected *statsRes
 }
 
 func checkSampleSliceDistributions(t *testing.T, samples []float64, nslices int, expected *statsResults) {
+	t.Helper()
 	chunk := len(samples) / nslices
 	for i := 0; i < nslices; i++ {
 		low := i * chunk