Update lua client to 0.0.1-24

The new version of the lua client uses libmpack as a backend, and some test
scripts had to be updated to reflect that.

test/functional/plugin/helpers.lua

@@ -25,7 +25,7 @@ local session = nil
 
 local reset = function(...)
   if session then
-    session:exit(0)
+    session:close()
   end
   session = spawn(nvim_argv(...))
   set_session(session)