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/GdUnitServer.tscn

@@ -0,0 +1,10 @@
+[gd_scene load_steps=3 format=3 uid="uid://cn5mp3tmi2gb1"]
+
+[ext_resource type="Script" path="res://addons/gdUnit4/src/network/GdUnitServer.gd" id="1"]
+[ext_resource type="Script" path="res://addons/gdUnit4/src/network/GdUnitTcpServer.gd" id="2"]
+
+[node name="Control" type="Node"]
+script = ExtResource("1")
+
+[node name="TcpServer" type="Node" parent="."]
+script = ExtResource("2")