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/update/GdUnitPatch.gd

@@ -0,0 +1,20 @@
+class_name GdUnitPatch
+extends RefCounted
+
+const PATCH_VERSION = "patch_version"
+
+var _version :GdUnit4Version
+
+
+func _init(version_ :GdUnit4Version) -> void:
+	_version = version_
+
+
+func version() -> GdUnit4Version:
+	return _version
+
+
+# this function needs to be implement
+func execute() -> bool:
+	push_error("The function 'execute()' is not implemented at %s" % self)
+	return false