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+.

runtime/lua/man.lua

@@ -1,4 +1,5 @@
 local buf_hls = {}
+local unpack = table.unpack or unpack -- luacheck: ignore
 
 local function highlight_line(line, linenr)
   local chars = {}