lnwire: update tests for init message

diff --git a/lnwire/init_message_test.go b/lnwire/init_message_test.go
new file mode 100644
index 0000000..d0a3aca
--- /dev/null
+++ b/lnwire/init_message_test.go
@@ -0,0 +1,61 @@
+package lnwire
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// TestInitEncodeDecode checks that we can encode and decode an Init message
+// to and from a byte stream.
+func TestInitEncodeDecode(t *testing.T) {
+	t.Parallel()
+
+	// These are the raw bytes that we expect to be generated from the
+	// sample Init message.
+	rawBytes := []byte{
+		// GlobalFeatures
+		0x00, 0x01, 0xc0,
+
+		// Features
+		0x00, 0x01, 0xc0,
+
+		// ExtraData - unknown odd-type TLV record.
+		0x6f,       // type (111)
+		0x02,       // length
+		0x79, 0x79, // value
+
+		// ExtraData - custom TLV record.
+		// TLV record for type 67676
+		0xfe, 0x00, 0x01, 0x08, 0x6c, // type (67676)
+		0x05,                         // length
+		0x01, 0x02, 0x03, 0x04, 0x05, // value
+
+		// ExtraData - custom TLV record.
+		// TLV record for type 67777
+		0xfe, 0x00, 0x01, 0x08, 0xc1, // type (67777)
+		0x03,             // length
+		0x01, 0x02, 0x03, // value
+	}
+
+	// Create a new empty message and decode the raw bytes into it.
+	msg := &Init{}
+	r := bytes.NewReader(rawBytes)
+	err := msg.Decode(r, 0)
+	require.NoError(t, err)
+
+	require.NotNil(t, msg.GlobalFeatures)
+	require.NotNil(t, msg.Features)
+	require.NotNil(t, msg.CustomRecords)
+	require.NotNil(t, msg.ExtraData)
+
+	// Next, encode the message back into a new byte buffer.
+	var b bytes.Buffer
+	err = msg.Encode(&b, 0)
+	require.NoError(t, err)
+
+	// The re-encoded bytes should be exactly the same as the original raw
+	// bytes.
+	require.Equal(t, rawBytes, b.Bytes())
+}
diff --git a/lnwire/test_message.go b/lnwire/test_message.go
index 8f946f1..0c2fe5e 100644
--- a/lnwire/test_message.go
+++ b/lnwire/test_message.go
@@ -1188,6 +1188,10 @@ func (msg *Init) RandTestMessage(t *rapid.T) Message {
 		local.Set(bit)
 	}
 
+	ignoreRecords := fn.NewSet[uint64]()
+
+	msg.ExtraData = RandExtraOpaqueData(t, ignoreRecords)
+
 	return NewInitMessage(global, local)
 }