chore(client): add gdUnit4 test framework addon

Vendor gdUnit4 for Godot 4 client testing (D-030).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/addons/gdUnit4/test/GdUnitScanTestSuitePerformanceTest.gd

@@ -0,0 +1,12 @@
+extends GdUnitTestSuite
+
+
+@warning_ignore("unsafe_method_access")
+func test_load_performance() -> void:
+	var time := LocalTime.now()
+	prints("Scan for test suites.")
+	var test_suites := GdUnitTestSuiteScanner.new().scan("res://addons/gdUnit4/test/")
+	assert_int(time.elapsed_since_ms())\
+		.override_failure_message("Expecting the loading time overall is less than 5s")\
+		.is_less(5*1000)
+	prints("Scanning of %d test suites took" % test_suites.size(), time.elapsed_since())