Remove some asserts in custom trigger test
Remove some asserts in custom trigger test

Since the CustomTrigger is tested in the other tests too, we only need
to test the C++ callback a few times.

joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue