unicode/utf16: add explicit test for decoding invalid runes.

The EncodeRune test exercises DecodeRune, but only for runes that it can encode. Add an explicit test for invalid utf16 surrogate pairs.

Bonus: coverage is now 100%

unicode/utf16/utf16.go: IsSurrogate     100.0%
unicode/utf16/utf16.go: DecodeRune      100.0%
unicode/utf16/utf16.go: EncodeRune      100.0%
unicode/utf16/utf16.go: Encode          100.0%
unicode/utf16/utf16.go: Decode          100.0%
total:                  (statements)    100.0%

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/39150044

src/pkg/unicode/utf16/utf16_test.go

@@ -100,6 +100,26 @@ func TestDecode(t *testing.T) {
 	}
 }
 
+var decodeRuneTests = []struct {
+	r1, r2 rune
+	want   rune
+}{
+	{0xd800, 0xdc00, 0x10000},
+	{0xd800, 0xdc01, 0x10001},
+	{0xd808, 0xdf45, 0x12345},
+	{0xdbff, 0xdfff, 0x10ffff},
+	{0xd800, 'a', 0xfffd}, // illegal, replacement rune substituted
+}
+
+func TestDecodeRune(t *testing.T) {
+	for i, tt := range decodeRuneTests {
+		got := DecodeRune(tt.r1, tt.r2)
+		if got != tt.want {
+			t.Errorf("%d: DecodeRune(%q, %q) = %v; want %v", i, tt.r1, tt.r2, got, tt.want)
+		}
+	}
+}
+
 var surrogateTests = []struct {
 	r    rune
 	want bool