test: Lua 5.2/5.3 compat #9280

Make the code run both on Lua 5.1 (which is the default for Neovim, and
is what LuaJIT provides) and Lua 5.2+.

test/functional/helpers.lua

@@ -2,6 +2,7 @@ require('coxpcall')
 local luv = require('luv')
 local lfs = require('lfs')
 local global_helpers = require('test.helpers')
+local unpack = table.unpack or unpack -- luacheck: ignore
 
 -- nvim client: Found in .deps/usr/share/lua/<version>/nvim/ if "bundled".
 local Session = require('nvim.session')