test(pack): adjust add startup tests to just sleep with longer timeout

Problem: The `add` startup tests mock startup process which requires
  *some* amount of time to be done. Previous solution with `vim.wait`
  to wait just enough time to register that startup script has finished
  doesn't seem to work as intended (it just wait full time). Its timeout
  is also seems to be barely enough to pass on Windows CI. Which will be
  a problem with more `git` actions done on startup in the future/next
  commit.

Solution: Just sleep predetermined amount of time and explicitly check
  if startup script finished executing.

test/functional/plugin/pack_spec.lua

@@ -828,7 +828,8 @@ describe('vim.pack', function()
       local function assert_works()
         -- Should auto-install but wait before executing code after it
         n.clear({ args_rm = { '-u' } })
-        n.exec_lua('vim.wait(500, function() return _G.done end, 50)')
+        vim.uv.sleep(2000)
+        eq(true, exec_lua('return _G.done'))
         assert_loaded()
 
         -- Should only `:packadd!`/`:packadd` already installed plugin