lnwire: add new TestMessage interface for property tests

In this commit, we add a new `TestMessage` interface for use in property
tests. With this, we'll be able to generate a random instance of a given
message, using the rapid byte stream. This can also eventually be useful
for fuzzing.

lnwire/shutdown.go

@@ -61,7 +61,8 @@ func NewShutdown(cid ChannelID, addr DeliveryAddress) *Shutdown {
 // interface.
 var _ Message = (*Shutdown)(nil)
 
-// A compile-time check to ensure Shutdown implements the lnwire.SizeableMessage interface.
+// A compile-time check to ensure Shutdown implements the lnwire.SizeableMessage
+// interface.
 var _ SizeableMessage = (*Shutdown)(nil)
 
 // Decode deserializes a serialized Shutdown from the passed io.Reader,