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/

addons/gdUnit4/bin/GdUnitCmdTool.gd

@@ -0,0 +1,21 @@
+#!/usr/bin/env -S godot -s
+extends SceneTree
+
+
+var _cli_runner: GdUnitTestCIRunner
+
+
+func _initialize() -> void:
+	DisplayServer.window_set_mode(DisplayServer.WINDOW_MODE_MINIMIZED)
+	_cli_runner = GdUnitTestCIRunner.new()
+	root.add_child(_cli_runner)
+
+
+# do not use print statements on _finalize it results in random crashes
+func _finalize() -> void:
+	queue_delete(_cli_runner)
+	if OS.is_stdout_verbose():
+		prints("Finallize ..")
+		prints("-Orphan nodes report-----------------------")
+		Window.print_orphan_nodes()
+		prints("Finallize .. done")