Fixing the examples (#7376)

Fixing a few failing examples, examples were referencing metrics that
were getting auto-generated in the past (like `sum__num`),
that we now prevent from getting created.

tests/core_tests.py

@@ -280,7 +280,8 @@ class CoreTests(SupersetTestCase):
             ]
         for name, method, url in urls:
             logging.info(f"[{name}]/[{method}]: {url}")
-            self.client.get(url)
+            resp = self.client.get(url)
+            self.assertEqual(resp.status_code, 200)
 
     def test_tablemodelview_list(self):
         self.login(username="admin")