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/GdUnitScanTestSuiteTest.gd

@@ -0,0 +1,29 @@
+# This test verifys only the scanner functionallity
+# to find all given tests by pattern 'func test_<testname>():'
+extends GdUnitTestSuite
+
+
+func test_example() -> void:
+	assert_str("This is an example message").has_length(26)
+	assert_str("This is an example message").starts_with("This is an ex")
+
+
+func test_b() -> void:
+	pass
+
+
+# test name starts with same name e.g. test_b vs test_b2
+func test_b2() -> void:
+	pass
+
+
+# test scanning success with invalid formatting
+func  	 test_b21 		 ( 	 ) 	 	  -> void:
+	pass
+
+
+# finally verify all tests are found
+func after() -> void:
+	assert_array(get_children())\
+		.extract("test_name")\
+		.contains_exactly(["test_example", "test_b", "test_b2", "test_b21"])