LibWeb: Add more document tests, add comment, text and mixin tests

Also adds a TypeScript definition file for the test runner object.

Libraries/LibWeb/CMakeLists.txt

@@ -34,6 +34,7 @@ set(SOURCES
     DOM/EventListener.cpp
     DOM/EventTarget.cpp
     DOM/Node.cpp
+    DOM/ParentNode.cpp
     DOM/Position.cpp
     DOM/TagNames.cpp
     DOM/Text.cpp