strings: fix off-by-one error in test

Previously it would panic because of out-of-bound access
if s1 is longer than s2.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/90110043

src/pkg/strings/strings_test.go

@@ -652,7 +652,7 @@ func equal(m string, s1, s2 string, t *testing.T) bool {
 	e1 := Split(s1, "")
 	e2 := Split(s2, "")
 	for i, c1 := range e1 {
-		if i > len(e2) {
+		if i >= len(e2) {
 			break
 		}
 		r1, _ := utf8.DecodeRuneInString(c1)