runtime/lua/vim/compat.lua

ref #9280
Introduce the `vim.compat` module, to help environments with system Lua
5.2+ run the build/tests. Include the module implicitly in all tests.

ref #8677
legacy `vim` module:
    beep
    buffer
    command
    dict
    eval
    firstline
    lastline
    line
    list
    open
    type
    window

test/functional/helpers.lua

@@ -1,8 +1,8 @@
+require('vim.compat')
 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')