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/helpers.lua

@@ -2,6 +2,7 @@ local assert = require('luassert')
 local luv = require('luv')
 local lfs = require('lfs')
 local relpath = require('pl.path').relpath
+local unpack = table.unpack or unpack -- luacheck: ignore
 
 local quote_me = '[^.%w%+%-%@%_%/]' -- complement (needn't quote)
 local function shell_quote(str)