djblets.auth.util.validate_test_cookie() was using an old module

location for ErrorList(), used for form errors. We had a couple

imports at the top of the module that were designed to locate the

correct version of this for validate_old_password, but

validate_test_cookie wasn't using it.
A more correct approach, which this change implements, is to use the

provided form's error_class attribute instead, which defaults to

ErrorList but could be something more specific on some forms. This

simplifies our code, since we don't have to guess where the class is,

and ensures we work with more form implementations.
Unit tests were added for the validation methods to ensure they work in

all Django versions.

Unit tests pass on Django 1.7 and 1.11.