Trial is not really a stand-alone unit test framework. It is an extension to unittest focused on the testing needs for the Twisted framework.
The Trial how-to http://twistedmatrix.com/projects/core/documentation/howto/testing.html has some information. More valuable, perhaps are the API documents http://twistedmatrix.com/documents/current/api/twisted.trial.html.
Because of the asynchronous nature of Twisted, the Fixture pattern has a rather complex relationship with the Twisted Reactor. Also, since Twisted is a framework, not a single application, components are optional, and the TestSuite pattern is implemented with considerably more flexibility.