math/big: fix TestBytes test

Fixes #12231.

Change-Id: I1f07c444623cd864667e21b2fee534eacdc193bb
Reviewed-on: https://go-review.googlesource.com/13814
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/math/big/int_test.go

@@ -387,6 +387,11 @@ func TestSetBytes(t *testing.T) {
 }
 
 func checkBytes(b []byte) bool {
+	// trim leading zero bytes since Bytes() won't return them
+	// (was issue 12231)
+	for len(b) > 0 && b[0] == 0 {
+		b = b[1:]
+	}
 	b2 := new(Int).SetBytes(b).Bytes()
 	return bytes.Equal(b, b2)
 }