Satisfy testlint.

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

test/functional/eval/string_spec.lua

@@ -8,6 +8,7 @@ local exc_exec = helpers.exc_exec
 local redir_exec = helpers.redir_exec
 local funcs = helpers.funcs
 local write_file = helpers.write_file
+local NIL = helpers.NIL
 
 describe('string() function', function()
   before_each(clear)