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 <[email protected]>
Reviewed-by: Mark Freeman <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Sean Liao <[email protected]>

Author: kolyshkin

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++ {
-			buf[i] = 0
-		}
+		clear(buf)
 	})
 }