sphinx_test: test jumbo size onion message packets

This commit adds a helper function to create onion messages of a
specified length. This helper is then used to test the handling of
packets larger than 1300 bytes specifically for onion messages.

Author: gijswijs

sphinx_test.go

@@ -846,6 +846,18 @@ func mustNewLegacyHopPayload(hopData *HopData) HopPayload {
 	return payload
 }
 
+// TestPaymentPathTotalPayloadSizeExceeds1300 tests that a PaymentPath can have
+// a TotalPayloadSize greater than 1300 bytes.
+func TestPaymentPathTotalPayloadSizeExceeds1300(t *testing.T) {
+	_, route, _, err := newOnionMessageRoute(15)
+	require.NoError(t, err, "newOnionMessageRoute should not return an "+
+		"error")
+
+	totalSize := route.TotalPayloadSize()
+	require.Greater(t, totalSize, 1300, "TotalPayloadSize should be "+
+		"greater than 1300")
+}
+
 // TestSphinxHopVariableSizedPayloads tests that we're able to fully decode an
 // EOB payload that was targeted at the final hop in a route, and also when
 // intermediate nodes have EOB data encoded as well. Additionally, we test that