Some recent unit tests for API token signals introduced a second

WebAPIToken model registered within the djblets.webapi.tests app.

While this didn't outright break on Django 1.6, it did on newer

versions.
These new tests now import the old model, allowing for a single

registered model. This works when running the entire Djblets test suite

or the individual test modules.
It also fixes a typo in the name of the class for the signal tests.

Ran webapi unit tests on Django 1.6, 1.9, 1.10, and 1.11 without

problems.