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/

.gitea/workflows/gitea-build.yaml

@@ -4,8 +4,10 @@
 #
 
 name: "Build"
-on: 
-  push: 
+on:
+  push:
+    branches: [main, develop]
+  pull_request:
     branches: [main, develop]
 
 env:
@@ -14,7 +16,24 @@ env:
   EXPORT_NAME: MusicPlayer
 
 jobs:
+  # Runs before build in the same workflow, so the two never race for the runner
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout Repository
+        uses: actions/checkout@v4
+      - name: Download Godot
+        run: |
+          curl -L -o godot.zip https://github.com/godotengine/godot-builds/releases/download/$GODOT_VERSION-stable/Godot_v$GODOT_VERSION-stable_linux.x86_64.zip
+          unzip godot.zip
+          mv Godot_v$GODOT_VERSION-stable_linux.x86_64 godot
+          chmod +x godot
+      - name: Run tests
+        run: GODOT=./godot ./run_tests.sh
+
   build:
+    needs: test
+    if: github.event_name == 'push'
     runs-on: ubuntu-latest
     steps:
       - name: Checkout Repository