Better diagnostics in tests in case text not found in response (#43428)

The assertions during tests checkign if content was in response,
did not print the content of the response, which made it difficult
to diagnose what was wrong with the response.

Adding response in case of failure should help to investigate
issues in CI where those asserts fail.

tests_common/test_utils/www.py

@@ -52,9 +52,9 @@ def check_content_in_response(text, resp, resp_code=200):
     assert resp_code == resp.status_code
     if isinstance(text, list):
         for line in text:
-            assert line in resp_html, f"Couldn't find {line!r}"
+            assert line in resp_html, f"Couldn't find {line!r}\nThe response was:\n{resp_html!r}"
     else:
-        assert text in resp_html, f"Couldn't find {text!r}"
+        assert text in resp_html, f"Couldn't find {text!r}\nThe response was:\n{resp_html!r}"
 
 
 def check_content_not_in_response(text, resp, resp_code=200):