Fixed django#18157 -- Documented that setting PASSWORD_HASHERS can speed up tests

Author: claudep

docs/topics/testing.txt

@@ -545,6 +545,18 @@ failed and erroneous tests. If all the tests pass, the return code is 0. This
 feature is useful if you're using the test-runner script in a shell script and
 need to test for success or failure at that level.
 
+Speeding up the tests
+---------------------
+
+In recent versions of Django, the default password hasher is rather slow by
+design. If during your tests you are authenticating many users, you may want
+to use a custom settings file and set the :setting:`PASSWORD_HASHERS` setting
+to a faster hashing algorithm::
+
+ PASSWORD_HASHERS = (
+ 'django.contrib.auth.hashers.MD5PasswordHasher',
+ )
+
 Testing tools
 =============