Replace the broken test suite with working GdUnit4 tests

The previous tests never compiled: they used GUT's assert functions,
Godot 3 APIs and reassigned autoload functions, so no test ran.

- 90 tests: unit tests for the playlist, songs, player scoring, time
  formatting, search and the server client, plus integration tests
  that run MainWindow and SyncWindow against a fake server
- Keybind tests check that Alt+<number> and Ctrl+Alt+<number> keep
  following the player rows while players are added and removed
- run_tests.sh finds Godot and runs the suite headless (~10 seconds)
- CI runs the tests before the build job in gitea-build.yaml; the
  separate test.yml used a GitHub action that does not exist
- GdUnit4 is committed instead of ignored, since the project enables
  it, and .gitea/ is no longer ignored
- Exported builds leave out tests/ and addons/gdUnit4/

tests/unit/test_song_object.gd

@@ -0,0 +1,60 @@
+extends GdUnitTestSuite
+
+var _is_debug: bool
+var _song: SongObject
+
+
+func before_test() -> void:
+    _is_debug = Settings.is_debug
+    Settings.is_debug = false
+    _song = SongObject.new()
+    _song.song_number = 4
+    _song.soundtrack_title = "Final Fantasy VI"
+    _song.song_title = "Dancing Mad"
+
+
+func after_test() -> void:
+    Settings.is_debug = _is_debug
+
+
+func test_titles_are_hidden_until_answered() -> void:
+    assert_str(_song.get_soundtrack_title()).is_equal("????????")
+    assert_str(_song.get_song_title()).is_equal("??????")
+    assert_str(_song.get_song_info()).is_equal("??? - ???")
+
+
+func test_titles_are_shown_once_answered() -> void:
+    _song.is_answered = true
+
+    assert_str(_song.get_soundtrack_title()).is_equal("Final Fantasy VI")
+    assert_str(_song.get_song_title()).is_equal("Dancing Mad")
+    assert_str(_song.get_song_info()).is_equal("5. Final Fantasy VI - Dancing Mad")
+
+
+func test_debug_info_shows_unanswered_songs() -> void:
+    Settings.is_debug = true
+
+    assert_str(_song.get_song_info()).is_equal(
+            "5. Final Fantasy VI - Dancing Mad | played: false | answered: false")
+
+
+func test_points_are_kept_in_the_order_given() -> void:
+    _song.add_point("marle-id")
+    _song.add_point("crono-id")
+
+    assert_array(_song.players_given_point).contains_exactly(["marle-id", "crono-id"])
+
+
+func test_player_gets_at_most_one_point_per_song() -> void:
+    _song.add_point("crono-id")
+    _song.add_point("crono-id")
+
+    assert_array(_song.players_given_point).contains_exactly(["crono-id"])
+
+
+func test_removing_point_of_player_without_point_changes_nothing() -> void:
+    _song.add_point("crono-id")
+
+    _song.remove_point("marle-id")
+
+    assert_array(_song.players_given_point).contains_exactly(["crono-id"])