Add unit testing in isolation article.

doc/xdocs/articles.html

@@ -46,6 +46,9 @@
     <li><b><a href="http://home.hccnet.nl/bart.van.riel/">Acegi Spring Tutorial</a></b>:
 		Available in PDF and HTML formats, thanks to Bart van Riel.<br><br>
 	</li>
+    <li><b><a href="http://peter.jteam.nl/wp-trackback.php?p=6">Testing Acegi Security</a></b>:
+		Peter Veentjer discussed how to test Acegi Security-protected objects in isolation.<br><br>
+	</li>
     <li><b><a href="http://dev.eclipse.org/mhonarc/lists/aspectj-users/msg05355.html">AspectJ with Acegi Security</a></b>:
 		AspectJ with Acegi Security thread on the AspectJ list.<br><br>
 	</li>