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/fakes/autoload_snapshot.gd

@@ -0,0 +1,27 @@
+class_name AutoloadSnapshot
+extends RefCounted
+## Saves and restores the script variables of autoloads such as Settings and Playlist,
+## so a test that changes global state does not leak into the next test.
+
+var _values: Dictionary = {}
+
+
+## Records the current values of every script variable on [param autoloads].
+static func take(autoloads: Array[Node]) -> AutoloadSnapshot:
+    var snapshot: AutoloadSnapshot = AutoloadSnapshot.new()
+    for autoload: Node in autoloads:
+        var values: Dictionary = {}
+        for property: Dictionary in autoload.get_property_list():
+            if property.usage & PROPERTY_USAGE_SCRIPT_VARIABLE:
+                var value: Variant = autoload.get(property.name)
+                values[property.name] = value.duplicate() if value is Array else value
+        snapshot._values[autoload] = values
+    return snapshot
+
+
+## Puts back the values recorded by [method take].
+func restore() -> void:
+    for autoload: Node in _values:
+        var values: Dictionary = _values[autoload]
+        for property_name: String in values:
+            autoload.set(property_name, values[property_name])