lnmock: add new package lnmock to host mocks

This commit introduces a new package to host mocked objects that can be
used for all the unit tests.

.golangci.yml

@@ -238,3 +238,10 @@ issues:
         - govet
         # itest case can be very long so we disable long function check.
         - funlen
+
+    - path: lnmock/*
+      linters:
+        # forcetypeassert is skipped for the mock because the test would fail
+        # if the returned value doesn't match the type, so there's no need to
+        # check the convert.
+        - forcetypeassert