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/fakes/main_window_test_suite.gd

@@ -0,0 +1,115 @@
+class_name MainWindowTestSuite
+extends GdUnitTestSuite
+## Base for tests that run the real main window against a fake MusicServer.
+
+const SONG_INFO: Array[Dictionary] = [
+    {"SongNo": 0, "Soundtrack": "Chrono Trigger", "Song": "Corridors of Time"},
+    {"SongNo": 1, "Soundtrack": "Final Fantasy VI", "Song": "Dancing Mad"},
+]
+
+var _server: FakeHttpTransport
+var _snapshot: AutoloadSnapshot
+var _runner: GdUnitSceneRunner
+var _window: Control
+
+
+func before_test() -> void:
+    _snapshot = AutoloadSnapshot.take([Settings, Playlist])
+    Playlist.reset_playlist()
+    Playlist.first_song_played = false
+    Playlist.is_song_downloading = false
+    Settings.player_array.clear()
+    Settings.add_to_stats = true
+    Settings.hide_next_track = true
+    Settings.use_low_played_mode = false
+    Settings.winning_score = 20
+
+    _server = FakeHttpTransport.new().install()
+    var mp3: PackedByteArray = FileAccess.get_file_as_bytes("res://tests/data/short.mp3")
+    _server.respond_json("/music/all", ["Chrono Trigger", "Final Fantasy VI"])
+    _server.respond_json("/music/all/order", ["Chrono Trigger", "Final Fantasy VI"])
+    _server.respond_json("/music/list", [])
+    _server.respond_json("/characters", [])
+    _server.respond_bytes("/music/rand", mp3)
+    _server.respond_bytes("/music/addQue", PackedByteArray())
+    _server.respond_json("/music/info", SONG_INFO[0])
+    _server.respond_bytes("/music/reset", PackedByteArray())
+    _server.respond_bytes("/music/played?song=0", PackedByteArray())
+
+    _runner = scene_runner("res://MainWindow.tscn")
+    _window = _runner.scene()
+    # The debug build lowers the winning score on startup; keep it out of reach
+    Settings.winning_score = 20
+
+
+func after_test() -> void:
+    # Let nodes queued for deletion by the window be freed before orphans are counted
+    await await_idle_frame()
+    _server.uninstall()
+    _snapshot.restore()
+
+
+## Makes the next song the server hands out the one described by [param info].
+func _next_song_is(info: Dictionary) -> void:
+    _server.respond_json("/music/info", info)
+
+
+func _press(button_path: String) -> void:
+    (_window.get_node(button_path) as Button).pressed.emit()
+
+
+## Presses Next and waits until the first song is playing.
+func _play_first_song() -> void:
+    _press("NextButton")
+    await _runner.simulate_frames(2)
+
+
+## Adds a player through the name field, the same way the user does.
+func _add_player(player_name: String) -> PlayerObject:
+    var name_field: TextEdit = _window.get_node(
+            "Players/VBoxContainer/AddPlayerContainer/PlayerNameField")
+    name_field.text = player_name
+    _runner.invoke("add_player")
+    return Settings.player_array.back() if !Settings.player_array.is_empty() else null
+
+
+## Returns the row in the player list that shows [param player_object].
+func _player_row(player_object: PlayerObject) -> Player:
+    var rows: Node = _window.get_node(
+            "Players/VBoxContainer/ScrollContainer/PlayersListVBoxContainer")
+    for row: Player in rows.get_children():
+        if row.player_object == player_object:
+            return row
+    return null
+
+
+## Removes a player with the remove button in its row.
+func _remove_player(player_object: PlayerObject) -> void:
+    (_player_row(player_object).get_node("HBoxContainer/RemovePlayer") as BaseButton) \
+            .pressed.emit()
+    await _runner.simulate_frames(1)
+
+
+## Presses [param keys] together, e.g. [code][KEY_ALT, KEY_1][/code], then releases them.
+func _press_keys(keys: Array[Key]) -> void:
+    for key: Key in keys:
+        _runner.simulate_key_press(key)
+    await _runner.await_input_processed()
+    keys.reverse()
+    for key: Key in keys:
+        _runner.simulate_key_release(key)
+    await _runner.await_input_processed()
+
+
+## Gives a point with Alt+<number>.
+func _give_point_with_keybind(number: int) -> void:
+    await _press_keys([KEY_ALT, (KEY_0 + number) as Key])
+
+
+## Takes a point with Ctrl+Alt+<number>.
+func _take_point_with_keybind(number: int) -> void:
+    await _press_keys([KEY_CTRL, KEY_ALT, (KEY_0 + number) as Key])
+
+
+func _scores() -> Array:
+    return Settings.player_array.map(func(p: PlayerObject) -> int: return p.player_score)