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/network/GdUnitTask.gd

@@ -0,0 +1,25 @@
+class_name GdUnitTask
+extends RefCounted
+
+const TASK_NAME = "task_name"
+const TASK_ARGS = "task_args"
+
+var _task_name :String
+var _fref :Callable
+
+
+func _init(task_name :String,instance :Object,func_name :String) -> void:
+	_task_name = task_name
+	if not instance.has_method(func_name):
+		push_error("Can't create GdUnitTask, Invalid func name '%s' for instance '%s'" % [instance, func_name])
+	_fref = Callable(instance, func_name)
+
+
+func name() -> String:
+	return _task_name
+
+
+func execute(args :Array) -> GdUnitResult:
+	if args.is_empty():
+		return _fref.call()
+	return _fref.callv(args)