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/custom.go

@@ -69,7 +69,7 @@ type Custom struct {
 	Data []byte
 }
 
-// A compile time check to ensure FundingCreated implements the lnwire.Message
+// A compile time check to ensure Custom implements the lnwire.Message
 // interface.
 var _ Message = (*Custom)(nil)