New Root URL: Disallow the use of real network requests to probe URLs…

… while running tests

src/crystal/url_input.py

@@ -83,17 +83,6 @@ def _candidate_urls_from_user_input(url_input: str) -> List[str]:
     * If additionally no www. domain prefix is given,
       try first without the prefix then with the prefix.
     """
-    # While running tests, unless CRYSTAL_URLOPEN_MOCKED=True,
-    # assume any URL input is already a valid URL without
-    # performing any (possibly real) network requests
-    if os.environ.get('CRYSTAL_RUNNING_TESTS', 'False') == 'True':
-        if os.environ.get('CRYSTAL_URLOPEN_MOCKED', 'False') == 'True':
-            # OK
-            pass
-        else:
-            # Assume valid, without performing any network requests
-            return [url_input]
-
     if url_input.strip() == '':
         return [url_input]
 
@@ -172,6 +161,23 @@ def _resolve_url_from_candidates(
     if len(url_candidates) == 1:
         return url_candidates[0]
 
+    # Disallow network requests while running tests,
+    # unless CRYSTAL_URLOPEN_MOCKED=True
+    if os.environ.get('CRYSTAL_RUNNING_TESTS', 'False') == 'True':
+        if os.environ.get('CRYSTAL_URLOPEN_MOCKED', 'False') == 'True':
+            # OK
+            pass
+        else:
+            raise AssertionError(
+                'Attempting to resolve URL candidates with real '
+                'network requests while automated tests are running. '
+
+                'Please either (1) enter URLs with http:// or https:// '
+                'schema that does not need to be resolved, or (2) '
+                'mock the urlopen() function with something like '
+                '_urlopen_responding_with().'
+            )
+
     # Look for URL candidate which can be fetched successfully
     for url_candidate in url_candidates:
         if did_cancel_func():