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/commit_sig.go

@@ -64,7 +64,8 @@ func NewCommitSig() *CommitSig {
 // interface.
 var _ Message = (*CommitSig)(nil)
 
-// A compile time check to ensure CommitSig implements the lnwire.SizeableMessage interface.
+// A compile time check to ensure CommitSig implements the
+// lnwire.SizeableMessage interface.
 var _ SizeableMessage = (*CommitSig)(nil)
 
 // Decode deserializes a serialized CommitSig message stored in the