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.

.gitignore

@@ -80,3 +80,5 @@ coverage.txt
 # Release build directory (to avoid build.vcs.modified Golang build tag to be
 # set to true by having untracked files in the working directory).
 /lnd-*/
+
+.aider*