[2.1.1] Add prefix directory to include path in standalone: this prevents PEAR from clobbering our unit tests

- Add missing include to unit test harness
- Add missing unit test for HTMLPurifier::getInstance

git-svn-id: http://htmlpurifier.org/svnroot/htmlpurifier/trunk@1358 48356398-32a2-884e-a903-53898d9a118a

tests/HTMLPurifierTest.php

@@ -132,5 +132,11 @@ alert("<This is compatible with XHTML>");
         );
     }
     
+    function testGetInstance() {
+        $purifier = HTMLPurifier::getInstance();
+        $purifier2 = HTMLPurifier::getInstance();
+        $this->assertReference($purifier, $purifier2);
+    }
+    
 }