unicode/utf8: add test for FullRune

Check that it now properly handles \xC0 and \xC1.

Fixes #11733.

Change-Id: I66cfe0d43f9d123d4c4509a3fa18b9b6380dfc39
Reviewed-on: https://go-review.googlesource.com/17225Reviewed-by: Russ Cox <rsc@golang.org>

src/unicode/utf8/utf8_test.go

@@ -100,6 +100,15 @@ func TestFullRune(t *testing.T) {
 			t.Errorf("FullRune(%q) = true, want false", s1)
 		}
 	}
+	for _, s := range []string{"\xc0", "\xc1"} {
+		b := []byte(s)
+		if !FullRune(b) {
+			t.Errorf("FullRune(%q) = false, want true", s)
+		}
+		if !FullRuneInString(s) {
+			t.Errorf("FullRuneInString(%q) = false, want true", s)
+		}
+	}
 }
 
 func TestEncodeRune(t *testing.T) {