strconv: strip \r in raw strings passed to Unquote

To match the language spec, strconv.Unquote needs to strip carriage
returns from the raw string.

Also fixes TestUnquote to not be a noop.

Fixes #15997

Change-Id: I2456f50f2ad3830f37e545f4f6774ced9fe609d7
Reviewed-on: https://go-review.googlesource.com/31210Reviewed-by: Robert Griesemer <gri@golang.org>

src/strconv/quote.go

@@ -362,6 +362,16 @@ func Unquote(s string) (string, error) {
 		if contains(s, '`') {
 			return "", ErrSyntax
 		}
+		if contains(s, '\r') {
+			// -1 because we know there is at least one \r to remove.
+			buf := make([]byte, 0, len(s)-1)
+			for i := 0; i < len(s); i++ {
+				if s[i] != '\r' {
+					buf = append(buf, s[i])
+				}
+			}
+			return string(buf), nil
+		}
 		return s, nil
 	}
 	if quote != '"' && quote != '\'' {

src/strconv/quote_test.go

@@ -274,6 +274,7 @@ var unquotetests = []unQuoteTest{
 	{"`\n`", "\n"},
 	{"`	`", `	`},
 	{"` `", ` `},
+	{"`a\rb`", "ab"},
 }
 
 var misquoted = []string{
@@ -306,7 +307,7 @@ var misquoted = []string{
 
 func TestUnquote(t *testing.T) {
 	for _, tt := range unquotetests {
-		if out, err := Unquote(tt.in); err != nil && out != tt.out {
+		if out, err := Unquote(tt.in); err != nil || out != tt.out {
 			t.Errorf("Unquote(%#q) = %q, %v want %q, nil", tt.in, out, err, tt.out)
 		}
 	}