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/GdUnitAssertImpl.gd

@@ -0,0 +1,78 @@
+class_name GdUnitAssertImpl
+extends GdUnitAssert
+
+
+var _current :Variant
+var _custom_failure_message :String = ""
+var _additional_failure_message: String = ""
+
+
+func _init(current :Variant) -> void:
+	_current = current
+	# save the actual assert instance on the current thread context
+	GdUnitThreadManager.get_current_context().set_assert(self)
+	GdAssertReports.reset_last_error_line_number()
+
+
+func current_value() -> Variant:
+	return _current
+
+
+func report_success() -> GdUnitAssert:
+	GdAssertReports.report_success()
+	return self
+
+
+func report_error(failure :String, failure_line_number: int = -1) -> GdUnitAssert:
+	var stack_trace := GdUnitStackTrace.new()
+	var line_number := failure_line_number if failure_line_number != -1 else stack_trace.get_line_number()
+	GdAssertReports.set_last_error_line_number(line_number)
+	var failure_message := GdAssertMessages.build_failure_message(failure, _additional_failure_message, _custom_failure_message)
+	GdAssertReports.report_error(GdUnitError.new(failure_message, line_number, stack_trace))
+	return self
+
+
+func do_fail_not_implemented() -> GdUnitAssert:
+	return report_error(GdAssertMessages.error_not_implemented())
+
+
+func do_fail_with_message(message: String) -> GdUnitAssert:
+	return report_error(GdAssertMessages.error_with_message(message))
+
+
+func override_failure_message(message: String) -> GdUnitAssert:
+	_custom_failure_message = message
+	return self
+
+
+func append_failure_message(message: String) -> GdUnitAssert:
+	_additional_failure_message = message
+	return self
+
+
+func is_null() -> GdUnitAssert:
+	var current :Variant = current_value()
+	if current != null:
+		return report_error(GdAssertMessages.error_is_null(current))
+	return report_success()
+
+
+func is_not_null() -> GdUnitAssert:
+	var current :Variant = current_value()
+	if current == null:
+		return report_error(GdAssertMessages.error_is_not_null())
+	return report_success()
+
+
+func is_equal(expected: Variant) -> GdUnitAssert:
+	var current: Variant = current_value()
+	if not GdObjects.equals(current, expected):
+		return report_error(GdAssertMessages.error_equal(current, expected))
+	return report_success()
+
+
+func is_not_equal(expected: Variant) -> GdUnitAssert:
+	var current: Variant = current_value()
+	if GdObjects.equals(current, expected):
+		return report_error(GdAssertMessages.error_not_equal(current, expected))
+	return report_success()