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/closing_signed.go

@@ -59,7 +59,8 @@ func NewClosingSigned(cid ChannelID, fs btcutil.Amount,
 // interface.
 var _ Message = (*ClosingSigned)(nil)
 
-// A compile time check to ensure ClosingSigned implements the lnwire.SizeableMessage interface.
+// A compile time check to ensure ClosingSigned implements the
+// lnwire.SizeableMessage interface.
 var _ SizeableMessage = (*ClosingSigned)(nil)
 
 // Decode deserializes a serialized ClosingSigned message stored in the passed