FIX VirtualPageTest failing on apps with no $db on Page

VirtualPageTest was adding fields to Page via a decorator, but Page might
not have any fields itself. This changed Page from not having a table
to having a table, which then caused a problem on tearDown

tests/model/VirtualPageTest.php

@@ -10,7 +10,7 @@ class VirtualPageTest extends SapphireTest {
 	);
 
 	protected $requiredExtensions = array(
-		'Page' => array('VirtualPageTest_PageExtension')
+		'SiteTree' => array('VirtualPageTest_PageExtension')
 	);
 
 	function setUp() {