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_complete.go

@@ -180,5 +180,6 @@ func (c *ClosingComplete) SerializedSize() (uint32, error) {
 // interface.
 var _ Message = (*ClosingComplete)(nil)
 
-// A compile time check to ensure ClosingComplete implements the lnwire.SizeableMessage interface.
+// A compile time check to ensure ClosingComplete implements the
+// lnwire.SizeableMessage interface.
 var _ SizeableMessage = (*ClosingComplete)(nil)