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_music_server_client.gd

@@ -0,0 +1,113 @@
+extends GdUnitTestSuite
+
+var _server: FakeHttpTransport
+
+
+func before_test() -> void:
+    _server = FakeHttpTransport.new().install()
+
+
+func after_test() -> void:
+    _server.uninstall()
+
+
+func test_fetch_song_list_passes_parsed_json() -> void:
+    var songs: Array = [{"SongNo": 0, "Soundtrack": "Chrono Trigger", "Song": "Corridors of Time"}]
+    _server.respond_json("/music/list", songs)
+    var received: Array = []
+
+    MusicServerClient.fetch_song_list(func(data: Array) -> void: received.append_array(data))
+
+    assert_int(received.size()).is_equal(1)
+    assert_int(int(received[0].SongNo)).is_equal(0)
+    assert_str(received[0].Soundtrack).is_equal("Chrono Trigger")
+
+
+func test_fetch_random_song_uses_normal_endpoint() -> void:
+    MusicServerClient.fetch_random_song(false, func(_body: PackedByteArray) -> void: pass)
+
+    assert_array(_server.requested_paths).contains_exactly(["/music/rand"])
+
+
+func test_fetch_random_song_uses_low_played_endpoint() -> void:
+    MusicServerClient.fetch_random_song(true, func(_body: PackedByteArray) -> void: pass)
+
+    assert_array(_server.requested_paths).contains_exactly(["/music/rand/low"])
+
+
+func test_fetch_song_passes_raw_bytes() -> void:
+    var mp3: PackedByteArray = PackedByteArray([0x49, 0x44, 0x33])
+    _server.respond_bytes("/music?song=4", mp3)
+    var received: Array[PackedByteArray] = []
+
+    MusicServerClient.fetch_song(4, func(body: PackedByteArray) -> void: received.append(body))
+
+    assert_array(received).contains_exactly([mp3])
+
+
+func test_failed_request_does_not_call_back() -> void:
+    _server.respond_json("/music/list", [], 500)
+    var called: Array[bool] = [false]
+
+    MusicServerClient.fetch_song_list(func(_data: Variant) -> void: called[0] = true)
+
+    assert_bool(called[0]).is_false()
+
+
+func test_invalid_json_does_not_call_back() -> void:
+    _server.respond_bytes("/music/info", "not json".to_utf8_buffer())
+    var called: Array[bool] = [false]
+
+    MusicServerClient.fetch_song_info(func(_data: Variant) -> void: called[0] = true)
+
+    assert_bool(called[0]).is_false()
+
+
+func test_send_calls_back_without_arguments() -> void:
+    _server.respond_bytes("/music/reset", PackedByteArray())
+    var called: Array[bool] = [false]
+
+    MusicServerClient.reset_playlist(func() -> void: called[0] = true)
+
+    assert_bool(called[0]).is_true()
+
+
+func test_mark_song_played_sends_put() -> void:
+    _server.respond_bytes("/music/played?song=3", PackedByteArray())
+
+    MusicServerClient.mark_song_played(3)
+
+    assert_array(_server.requested_paths).contains_exactly(["/music/played?song=3"])
+    assert_array(_server.requested_methods).contains_exactly([HTTPClient.METHOD_PUT])
+
+
+func test_character_name_is_url_encoded() -> void:
+    MusicServerClient.fetch_character_image("Paper Mario.png", func(_body: PackedByteArray) -> void: pass)
+
+    assert_array(_server.requested_paths).contains_exactly(["/character?name=Paper%20Mario.png"])
+
+
+func test_requests_go_to_selected_server() -> void:
+    var previous_path: String = Settings.default_path
+    Settings.default_path = "http://localhost:8080"
+    var requested_urls: Array[String] = []
+    var recording_transport: HttpTransport = _RecordingTransport.new(requested_urls)
+    MusicServerClient.transport = recording_transport
+
+    MusicServerClient.fetch_song_info(func(_data: Variant) -> void: pass)
+
+    Settings.default_path = previous_path
+    assert_array(requested_urls).contains_exactly(["http://localhost:8080/music/info"])
+
+
+class _RecordingTransport:
+    extends HttpTransport
+
+    var _urls: Array[String]
+
+    func _init(urls: Array[String]) -> void:
+        super(null)
+        _urls = urls
+
+    func request(url: String, _method: int, _on_complete: Callable) -> void:
+        _urls.append(url)