bytes: use clear in test

Replace for loop with clear built-in, available since Go 1.21.

Change-Id: I66d0124b9004ffd0b52abb679ccff86bb600c1e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/704878
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sean Liao <sean@liao.dev>

src/bytes/bytes_test.go

@@ -891,9 +891,7 @@ func BenchmarkCountSingle(b *testing.B) {
 				b.Fatal("bad count", j, expect)
 			}
 		}
-		for i := 0; i < len(buf); i++ {
+		clear(buf)
-			buf[i] = 0
-		}
 	})
 }