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/

tests/unit/test_settings.gd

@@ -1,94 +0,0 @@
-class_name SettingsTest
-extends GdUnitTestSuite
-
-
-func test_make_request2_calls_callback_on_success():
-	var callback_called = false
-	var received_data = null
-	
-	var callback = func(data):
-		callback_called = true
-		received_data = data
-	
-	# Mock the HTTP request behavior
-	var original_make_request = Settings.make_request2
-	Settings.make_request2 = func(address: String, func_name: Callable, expect_data: bool) -> void:
-		if expect_data:
-			func_name.call([{"SongNo": 1, "Soundtrack": "Test", "Song": "Test Song"}])
-		else:
-			func_name.call()
-	
-	Settings.make_request2("/music/list", callback, true)
-	
-	assert_true(callback_called)
-	assert_eq(received_data[0].SongNo, 1)
-	
-	# Restore original
-	Settings.make_request2 = original_make_request
-
-
-func test_make_request2_handles_error_response():
-	var error_called = false
-	
-	var callback = func(data):
-		# This shouldn't be called for error responses
-		pass
-	
-	# Mock error response
-	var original_make_request = Settings.make_request2
-	Settings.make_request2 = func(address: String, func_name: Callable, expect_data: bool) -> void:
-		# Simulate error by not calling the callback
-		pass
-	
-	Settings.make_request2("/music/error", callback, true)
-	
-	# Restore and verify callback wasn't called with data
-	Settings.make_request2 = original_make_request
-	assert_false(error_called)
-
-
-func test_make_request3_does_not_expect_data():
-	var callback_called = false
-	
-	var callback = func():
-		callback_called = true
-	
-	var original_make_request = Settings.make_request3
-	Settings.make_request3 = func(address: String) -> void:
-		callback.call()
-	
-	Settings.make_request3("/music/reset")
-	
-	assert_true(callback_called)
-	
-	Settings.make_request3 = original_make_request
-
-
-func test_delete_children_removes_all():
-	var node = Node.new()
-	var child1 = Node.new()
-	var child2 = Node.new()
-	
-	node.add_child(child1)
-	node.add_child(child2)
-	
-	assert_eq(node.get_child_count(), 2)
-	
-	Settings.delete_children(node)
-	
-	assert_eq(node.get_child_count(), 0)
-
-
-func test_delete_player_children_removes_all():
-	var node = Node.new()
-	var player1 = Player.new()
-	var player2 = Player.new()
-	
-	node.add_child(player1)
-	node.add_child(player2)
-	
-	assert_eq(node.get_child_count(), 2)
-	
-	Settings.delete_player_children(node)
-	
-	assert_eq(node.get_child_count(), 0)