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/channel_update.go

@@ -124,7 +124,8 @@ type ChannelUpdate1 struct {
 // interface.
 var _ Message = (*ChannelUpdate1)(nil)
 
-// A compile time check to ensure ChannelUpdate1 implements the lnwire.SizeableMessage interface.
+// A compile time check to ensure ChannelUpdate1 implements the
+// lnwire.SizeableMessage interface.
 var _ SizeableMessage = (*ChannelUpdate1)(nil)
 
 // Decode deserializes a serialized ChannelUpdate stored in the passed