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/src/monitor/GdUnitMonitor.gd

@@ -0,0 +1,24 @@
+# GdUnit Monitoring Base Class
+class_name GdUnitMonitor
+extends RefCounted
+
+var _id :String
+
+# constructs new Monitor with given id
+func _init(p_id :String) -> void:
+	_id = p_id
+
+
+# Returns the id of the monitor to uniqe identify
+func id() -> String:
+	return _id
+
+
+# starts monitoring
+func start() -> void:
+	pass
+
+
+# stops monitoring
+func stop() -> void:
+	pass