Fix tests after adding key to settings

tests/test_posts.py

     with open(os.path.join(temp_dir.name, 'settings.json'), "w") as f:
         f.write(json.dumps({
             "PUBLIC_PATH": os.path.join(temp_dir.name, 'public'),
-            "SITE_NAME": "test website"
+            "SITE_NAME": "test website",
+            "MARKDOWN_PREVIEW": True
         }))
 
     # Importing PostsService initializes a Settings object which depends on

tests/test_settings.py

     with open(os.path.join(root, "settings.json"), "w") as f:
         f.write(json.dumps({
             "PUBLIC_PATH": os.path.join(root, "public"),
-            "SITE_NAME": "test website"
+            "SITE_NAME": "test website",
+            "MARKDOWN_PREVIEW": True
         }))
     assert Settings().YIRD_PATH == root
     shutil.rmtree(root)