deleting AddToTestCaseRegistry and related

The goal of finding all the Simulator test classes can be achieved using
TestLoader.discover, which is provided by unittest.
helpers.simulator_suite doesn't import *all* tests

It should only import SimulatorTestCase subclasses, and now does. At least via
nosetests.  nengo_ocl's unit tests don't work using unittest discover because
the TestLoader.discover call refuses to discover tests within a foreign
project. Still this is better than what was before.