LibWeb+test-web: Create test-web program, add doctype test

LibWeb currently has no test suite or program. Let's change that :^)

test-web is mostly a copy of test-js, but modified for LibWeb.
test-web imports both LibJS/Tests/test-common.js and
LibWeb/Test/test-common.js

LibWeb's suite provides the ability to specify the page to load,
what to do before the page is loaded, and what to do after it's
loaded.

This also provides a test of document.doctype and its close sibling
document.compatMode.

Currently, this isn't added to Lagom because of CodeGenerators.

Libraries/LibWeb/Tests/test-common.js

@@ -0,0 +1,28 @@
+// NOTE: The tester loads in LibJS's test-common to prevent duplication.
+
+let __PageToLoad__;
+
+// This tells the tester which page to load.
+// This will only be checked when we look at which page the test wants to use.
+// Subsequent calls to loadPage in before/after load will be ignored.
+let loadPage;
+
+let __BeforePageLoad__ = () => {};
+
+// This function will be run just before loading the page.
+// This is useful for injecting event listeners.
+// Defaults to an empty function.
+let beforePageLoad;
+
+let __AfterPageLoad__ = () => {};
+
+// This function will be run just after loading the page.
+// This is where the main bulk of the tests should be.
+// Defaults to an empty function.
+let afterPageLoad;
+
+(() => {
+    loadPage = (page) => __PageToLoad__ = page;
+    beforePageLoad = (callback) => __BeforePageLoad__ = callback;
+    afterPageLoad = (callback) => __AfterPageLoad__ = callback;
+})();