Add a test for false-positive warnings about intersite links.

With the current implementation, links to articles/pages that have not
been processed yet are reported with a warning, yet later replaced with
a correct value. That's not wanted.

pelican/tests/cyclic_intersite_links/third-article.rst

@@ -0,0 +1,7 @@
+Third article
+#############
+
+:date: 2018-11-10
+:summary: Here's the `first <{filename}/first-article.rst>`_,
+    `second <{filename}/second-article.rst>`_ and a
+    `nonexistent article <{filename}/nonexistent.rst>`_.