btcec: Add case to signature serialization test

It adds the case where the S value of the signature is bigger than the
half of the order of the curve.

btcec/signature_test.go

@@ -417,6 +417,24 @@ func TestSignatureSerialize(t *testing.T) {
 				0x2e, 0xdd, 0xc2, 0x1c, 0x14, 0x15, 0xac,
 			},
 		},
+		{
+			"valid 4 - s is bigger than half order",
+			&Signature{
+				R: fromHex("a196ed0e7ebcbe7b63fe1d8eecbdbde03a67ceba4fc8f6482bdcb9606a911404"),
+				S: fromHex("971729c7fa944b465b35250c6570a2f31acbb14b13d1565fab7330dcb2b3dfb1"),
+			},
+			[]byte{
+				0x30, 0x45, 0x02, 0x21, 0x00, 0xa1, 0x96, 0xed,
+				0x0e, 0x7e, 0xbc, 0xbe, 0x7b, 0x63, 0xfe, 0x1d,
+				0x8e, 0xec, 0xbd, 0xbd, 0xe0, 0x3a, 0x67, 0xce,
+				0xba, 0x4f, 0xc8, 0xf6, 0x48, 0x2b, 0xdc, 0xb9,
+				0x60, 0x6a, 0x91, 0x14, 0x04, 0x02, 0x20, 0x68,
+				0xe8, 0xd6, 0x38, 0x05, 0x6b, 0xb4, 0xb9, 0xa4,
+				0xca, 0xda, 0xf3, 0x9a, 0x8f, 0x5d, 0x0b, 0x9f,
+				0xe3, 0x2b, 0x9b, 0x9b, 0x77, 0x49, 0xdc, 0x14,
+				0x5f, 0x2d, 0xb0, 0x1d, 0x82, 0x61, 0x90,
+			},
+		},
 		{
 			"zero signature",
 			&Signature{