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/stfu.go

@@ -24,7 +24,8 @@ type Stfu struct {
 // A compile time check to ensure Stfu implements the lnwire.Message interface.
 var _ Message = (*Stfu)(nil)
 
-// A compile time check to ensure Stfu implements the lnwire.SizeableMessage interface.
+// A compile time check to ensure Stfu implements the lnwire.SizeableMessage
+// interface.
 var _ SizeableMessage = (*Stfu)(nil)
 
 // Encode serializes the target Stfu into the passed io.Writer.