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/node_announcement.go

@@ -104,7 +104,8 @@ type NodeAnnouncement struct {
 // lnwire.Message interface.
 var _ Message = (*NodeAnnouncement)(nil)
 
-// A compile time check to ensure NodeAnnouncement implements the lnwire.SizeableMessage interface.
+// A compile time check to ensure NodeAnnouncement implements the
+// lnwire.SizeableMessage interface.
 var _ SizeableMessage = (*NodeAnnouncement)(nil)
 
 // Decode deserializes a serialized NodeAnnouncement stored in the passed