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/ui/settings/GdUnitInputCapture.tscn

@@ -0,0 +1,36 @@
+[gd_scene load_steps=2 format=3 uid="uid://pmnkxrhglak5"]
+
+[ext_resource type="Script" path="res://addons/gdUnit4/src/ui/settings/GdUnitInputCapture.gd" id="1_gki1u"]
+
+[node name="GdUnitInputMapper" type="Control"]
+modulate = Color(0.929099, 0.929099, 0.929099, 0.936189)
+top_level = true
+layout_mode = 3
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+grow_horizontal = 2
+grow_vertical = 2
+size_flags_horizontal = 3
+size_flags_vertical = 3
+script = ExtResource("1_gki1u")
+
+[node name="Label" type="Label" parent="."]
+unique_name_in_owner = true
+self_modulate = Color(0.401913, 0.401913, 0.401913, 0.461723)
+top_level = true
+layout_mode = 1
+anchors_preset = 8
+anchor_left = 0.5
+anchor_top = 0.5
+anchor_right = 0.5
+anchor_bottom = 0.5
+offset_left = -60.5
+offset_top = -19.5
+offset_right = 60.5
+offset_bottom = 19.5
+grow_horizontal = 2
+grow_vertical = 2
+theme_override_colors/font_color = Color(1, 1, 1, 1)
+theme_override_font_sizes/font_size = 26
+text = "Press keys for shortcut"