runtime: make use of builtin clear in tests

This is a follow-up to CL 574675.

Change-Id: I98c3ea968e9c7dc61472849c385a1e697568aa30
Reviewed-on: https://go-review.googlesource.com/c/go/+/574975
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Jes Cok <xigua67damn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

src/runtime/hash_test.go

@@ -339,9 +339,7 @@ type BytesKey struct {
 }
 
 func (k *BytesKey) clear() {
-	for i := range k.b {
-		k.b[i] = 0
-	}
+	clear(k.b)
 }
 func (k *BytesKey) random(r *rand.Rand) {
 	randBytes(r, k.b)

src/runtime/memmove_test.go

@@ -451,9 +451,7 @@ func BenchmarkGoMemclr(b *testing.B) {
 	benchmarkSizes(b, []int{5, 16, 64, 256}, func(b *testing.B, n int) {
 		x := make([]byte, n)
 		for i := 0; i < b.N; i++ {
-			for j := range x {
-				x[j] = 0
-			}
+			clear(x)
 		}
 	})
 }