Fix connected_players on_shutdown

[cx384]

Goal

Fixes #14736

How it works:

• Reorderes what happens in Server::~Server().
  (It should be safe, to first stop the ServerThread before kicking the players, as far as I can tell.
  As before, the EmergeManager threads are stopped before the on_shutdown callback happens.)
• Adds a test to devtest.
  (I'm not sure if I added it at the right place, but it is similar to the mods/unittests/load_time.lua file.)

To do

This PR is Ready for Review.

How to test

Start devtest and exit devtest.
(The test gets executed every time, similar to what happens in mods/unittests/load_time.lua.)

[SmallJoker]

This looks good and works from what I've tested so far (that is: saving player metadata in the on_shutdown callback).

Would you please be so nice to mention this behaviour in the on_shutdown Lua API documentation? Suggestion:

* Players that were kicked by the shutdown procedure are still fully accessible
  in `minetest.get_connected_players()` .

[cx384]

I added it to the on_shutdown Lua API documentation.

[appgurueu]

Tested, works. Code changes make sense. Thanks.

[sfan5]

seems ok

[appgurueu]

@SmallJoker do you want to give this another look or test, or are you fine with it as-is?