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/asserts/GdUnitBoolAssertImpl.gd

@@ -0,0 +1,83 @@
+extends GdUnitBoolAssert
+
+var _base: GdUnitAssertImpl
+
+
+func _init(current :Variant) -> void:
+	_base = GdUnitAssertImpl.new(current)
+	# save the actual assert instance on the current thread context
+	GdUnitThreadManager.get_current_context().set_assert(self)
+	if not GdUnitAssertions.validate_value_type(current, TYPE_BOOL):
+		@warning_ignore("return_value_discarded")
+		report_error("GdUnitBoolAssert inital error, unexpected type <%s>" % GdObjects.typeof_as_string(current))
+
+
+func _notification(event :int) -> void:
+	if event == NOTIFICATION_PREDELETE:
+		if _base != null:
+			_base.notification(event)
+			_base = null
+
+
+func current_value() -> Variant:
+	return _base.current_value()
+
+
+func report_success() -> GdUnitBoolAssert:
+	@warning_ignore("return_value_discarded")
+	_base.report_success()
+	return self
+
+
+func report_error(error :String) -> GdUnitBoolAssert:
+	@warning_ignore("return_value_discarded")
+	_base.report_error(error)
+	return self
+
+
+func override_failure_message(message: String) -> GdUnitBoolAssert:
+	@warning_ignore("return_value_discarded")
+	_base.override_failure_message(message)
+	return self
+
+
+func append_failure_message(message: String) -> GdUnitBoolAssert:
+	@warning_ignore("return_value_discarded")
+	_base.append_failure_message(message)
+	return self
+
+
+func is_null() -> GdUnitBoolAssert:
+	@warning_ignore("return_value_discarded")
+	_base.is_null()
+	return self
+
+
+func is_not_null() -> GdUnitBoolAssert:
+	@warning_ignore("return_value_discarded")
+	_base.is_not_null()
+	return self
+
+
+func is_equal(expected: Variant) -> GdUnitBoolAssert:
+	@warning_ignore("return_value_discarded")
+	_base.is_equal(expected)
+	return self
+
+
+func is_not_equal(expected: Variant) -> GdUnitBoolAssert:
+	@warning_ignore("return_value_discarded")
+	_base.is_not_equal(expected)
+	return self
+
+
+func is_true() -> GdUnitBoolAssert:
+	if current_value() != true:
+		return report_error(GdAssertMessages.error_is_true(current_value()))
+	return report_success()
+
+
+func is_false() -> GdUnitBoolAssert:
+	if current_value() == true || current_value() == null:
+		return report_error(GdAssertMessages.error_is_false(current_value()))
+	return report_success()