LibWeb+WebContent+headless-browser: Support async text tests

Previously, we used `on_load_finish` to determine when the text test
was completed. This method did not allow testing of async functions
because there was no way to indicate that the runner should wait for
the async call to end.

This change introduces a function in the `internals` object that is
intended to be called when the text test execution is completed. The
text test runner will now ignore `on_load_finish` which means a test
will timeout if this new function is never called.

`test(f)` function in `include.js` has been modified to automatically
terminate a test once `load` event is fired on `window`.
new `asyncTest(f)` function has been introduces. `f` receives function
that will terminate a test as a first argument.

Every test is expected to call either `test()` or `asyncTest()` to
complete. If not, it will remain hanging until a timeout occurs.

Userland/Utilities/headless-browser.cpp

@@ -196,8 +196,10 @@ static ErrorOr<TestResult> run_dump_test(HeadlessWebContentView& view, StringVie
             result = builder.to_string().release_value_but_fixme_should_propagate_errors();
             loop.quit(0);
         };
+        view.on_text_test_finish = {};
     } else if (mode == TestMode::Text) {
-        view.on_load_finish = [&](auto const&) {
+        view.on_load_finish = {};
+        view.on_text_test_finish = [&]() {
             result = view.dump_text().release_value_but_fixme_should_propagate_errors();
             loop.quit(0);
         };