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/GdUnitTcpNode.gd

@@ -0,0 +1,55 @@
+class_name GdUnitTcpNode
+extends Node
+
+## Maximum size of a single RPC payload. Godot's default of 64 KiB is too small for larger test
+## events (e.g. reports with long stack traces), so it is raised here. Kept modest because
+## `PacketPeerStream` allocates its buffers eagerly at this size, per connection.
+const MAX_PACKET_SIZE := 2 * 1024 * 1024
+
+# `PacketPeerStream` reassembles length-prefixed packets from a raw `StreamPeerTCP` byte stream,
+# buffering partial reads across calls instead of assuming a full message always arrives at once.
+var _packets := _new_packet_peer()
+
+
+static func _new_packet_peer() -> PacketPeerStream:
+	var packet_peer := PacketPeerStream.new()
+	packet_peer.input_buffer_max_size = MAX_PACKET_SIZE
+	packet_peer.output_buffer_max_size = MAX_PACKET_SIZE
+	return packet_peer
+
+
+func rpc_send(stream: StreamPeerTCP, data: RPC) -> void:
+	_packets.stream_peer = stream
+	var payload := data.serialize().to_utf16_buffer()
+	var status_code := _packets.put_packet(payload)
+	if status_code != OK:
+		push_error("'rpc_send:' Can't put_packet(), error: %s" % error_string(status_code))
+
+
+func receive_packages(stream: StreamPeerTCP, rpc_cb: Callable = noop) -> Array[RPC]:
+	var received_packages: Array[RPC] = []
+	if stream.get_status() != StreamPeerTCP.STATUS_CONNECTED:
+		return received_packages
+
+	_packets.stream_peer = stream
+	while _packets.get_available_packet_count() > 0:
+		var payload := _packets.get_packet()
+		var status_code := _packets.get_packet_error()
+		if status_code != OK:
+			push_error("'receive_packages:' Can't get_packet(), error: %s" % error_string(status_code))
+			continue
+
+		var json := payload.get_string_from_utf16()
+		if json.is_empty():
+			push_warning("json is empty, can't process data")
+			continue
+		var data := RPC.deserialize(json)
+		if data == null:
+			continue
+		received_packages.append(data)
+		rpc_cb.call(data)
+	return received_packages
+
+
+static func noop(_rpc_data: RPC) -> void:
+	pass