Update unit tests to use XML as well.

Danbooru and Konachan aren't passing just because the posts retrieved
aren't the same.

src/libdanbooru/autotests/test_danbooru_post.h

@@ -35,6 +35,8 @@ private Q_SLOTS:
     void initTestCase();
     void testPostLoading();
     void testPostLoading_data();
+    void testPostLoadingXml();
+    void testPostLoadingXml_data();
 
 };