Fixed #23289 -- Added mock as a test dependency.

tests/runtests.py

@@ -374,6 +374,16 @@ if __name__ == "__main__":
         help='Run the Selenium tests as well (if Selenium is installed)')
     options = parser.parse_args()
 
+    # mock is a required dependency
+    try:
+        from django.test import mock  # NOQA
+    except ImportError:
+        print(
+            "Please install test dependencies first: \n"
+            "$ pip install -r requirements/py%s.txt" % sys.version_info.major
+        )
+        sys.exit(1)
+
     # Allow including a trailing slash on app_labels for tab completion convenience
     options.modules = [os.path.normpath(labels) for labels in options.modules]