LibJS: Skip test262 tests with the CanBlockIsFalse flag

From test262 documentation, this flag means:

    The test file should only be run when the [[CanBlock]] property of
    the Agent Record executing the file is `false`.

This patch stubs out the accessor for that internal slot and skips tests
with the CanBlockIsFalse if that internal slot is true.

Userland/Libraries/LibJS/Runtime/Agent.h

@@ -0,0 +1,13 @@
+/*
+ * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+namespace JS {
+
+bool agent_can_suspend();
+
+}