Satisfy testlint.

For that, make luatest ignore the preload.lua files.

test/functional/plugin/msgpack_spec.lua

@@ -3,6 +3,7 @@ local meths = helpers.meths
 local eq, nvim_eval, nvim_command, exc_exec =
   helpers.eq, helpers.eval, helpers.command, helpers.exc_exec
 local ok = helpers.ok
+local NIL = helpers.NIL
 
 local plugin_helpers = require('test.functional.plugin.helpers')
 local reset = plugin_helpers.reset