Enable lj-releng tool to lint lua code.

rootfs/etc/nginx/lua/test/run.lua

@@ -10,7 +10,6 @@ do
 
   -- if there's more constants need to be whitelisted for test runs, add here.
   local GLOBALS_ALLOWED_IN_TEST = {
-    _TEST = true,
     helpers = true,
   }
   local newindex = function(table, key, value)
@@ -35,7 +34,6 @@ do
 end
 
 _G.helpers = require("test.helpers")
-_G._TEST = true
 
 local ffi = require("ffi")
 local lua_ingress = require("lua_ingress")