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/

export_presets.cfg

@@ -8,7 +8,7 @@ dedicated_server=false
 custom_features=""
 export_filter="all_resources"
 include_filter=""
-exclude_filter=""
+exclude_filter="tests/*, addons/gdUnit4/*"
 export_path="../../MusicPlayer_1.5.1.exe"
 patches=PackedStringArray()
 encryption_include_filters=""
@@ -83,7 +83,7 @@ dedicated_server=false
 custom_features=""
 export_filter="all_resources"
 include_filter=""
-exclude_filter=""
+exclude_filter="tests/*, addons/gdUnit4/*"
 export_path="./MusicPlayer_0.9_Beta.dmg"
 patches=PackedStringArray()
 encryption_include_filters=""
@@ -343,7 +343,7 @@ dedicated_server=false
 custom_features=""
 export_filter="all_resources"
 include_filter=""
-exclude_filter=""
+exclude_filter="tests/*, addons/gdUnit4/*"
 export_path="web/index.html"
 patches=PackedStringArray()
 encryption_include_filters=""
@@ -390,7 +390,7 @@ dedicated_server=false
 custom_features=""
 export_filter="all_resources"
 include_filter=""
-exclude_filter=""
+exclude_filter="tests/*, addons/gdUnit4/*"
 export_path="./MusicPlayer_1.5.1.x86_64"
 patches=PackedStringArray()
 encryption_include_filters=""