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/api/window_spec.lua

@@ -12,6 +12,7 @@ local meth_pcall = helpers.meth_pcall
 local meths = helpers.meths
 local command = helpers.command
 local expect_err = helpers.expect_err
+local unpack = table.unpack or unpack -- luacheck: ignore
 
 -- check if str is visible at the beginning of some line
 local function is_visible(str)