lnrpc: add test case to TestExtractIntentFromSendRequest

Here we add a test case to TestExtractIntentFromSendRequest which shows
that an error is returned if the destination feature bit vector of a
payment request contains both the required and optional bits for a given
feature. This will be updated in an upcoming commit to be less strict.

lnrpc/routerrpc/router_backend_test.go

@@ -809,6 +809,28 @@ func TestExtractIntentFromSendRequest(t *testing.T) {
 			valid:            false,
 			expectedErrorMsg: "self-payments not allowed",
 		},
+		{
+			name: "Required and optional feature bits set",
+			backend: &RouterBackend{
+				MaxTotalTimelock: 1000,
+				ShouldSetExpEndorsement: func() bool {
+					return false
+				},
+			},
+			sendReq: &SendPaymentRequest{
+				Dest:             destNodeBytes,
+				Amt:              int64(paymentAmount),
+				PaymentHash:      make([]byte, 32),
+				MaxParts:         10,
+				MaxShardSizeMsat: 30_000_000,
+				DestFeatures: []lnrpc.FeatureBit{
+					lnrpc.FeatureBit_GOSSIP_QUERIES_OPT,
+					lnrpc.FeatureBit_GOSSIP_QUERIES_REQ,
+				},
+			},
+			valid:            false,
+			expectedErrorMsg: "feature pair exists",
+		},
 		{
 			name: "Valid send req parameters, payment settled",
 			backend: &RouterBackend{