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/fake_http_transport.gd

@@ -0,0 +1,56 @@
+class_name FakeHttpTransport
+extends HttpTransport
+## Stands in for the MusicServer in tests.
+##
+## Register canned responses per path with [method respond_json] or
+## [method respond_bytes], then install it with [method install]. Requests to paths
+## without a response get a 404. Responses are delivered immediately.
+
+## Paths requested so far, relative to [code]Settings.default_path[/code].
+var requested_paths: Array[String] = []
+## HTTP method of each request, in the same order as [member requested_paths].
+var requested_methods: Array[int] = []
+
+var _responses: Dictionary = {}
+var _previous_transport: HttpTransport
+
+
+func _init() -> void:
+    super(null)
+
+
+## Replaces the MusicServerClient transport with this fake.
+func install() -> FakeHttpTransport:
+    _previous_transport = MusicServerClient.transport
+    MusicServerClient.transport = self
+    return self
+
+
+## Restores the transport that was active before [method install].
+func uninstall() -> void:
+    MusicServerClient.transport = _previous_transport
+
+
+## Answers requests to [param path] with [param data] encoded as JSON.
+func respond_json(path: String, data: Variant, response_code: int = 200) -> void:
+    respond_bytes(path, JSON.stringify(data).to_utf8_buffer(), response_code)
+
+
+## Answers requests to [param path] with [param body] as-is.
+func respond_bytes(path: String, body: PackedByteArray, response_code: int = 200) -> void:
+    _responses[path] = {"code": response_code, "body": body}
+
+
+## Returns how many times [param path] has been requested.
+func count(path: String) -> int:
+    return requested_paths.count(path)
+
+
+func request(url: String, method: int, on_complete: Callable) -> void:
+    var path: String = url.trim_prefix(Settings.default_path)
+    requested_paths.append(path)
+    requested_methods.append(method)
+    if _responses.has(path):
+        on_complete.call(_responses[path].code, _responses[path].body)
+    else:
+        on_complete.call(404, PackedByteArray())