[tests] rewrite pickling test using basicinterface_subclasses fixture
[tests] do not inherit from BasicInterface in pymortests

Otherwise such classes will be collected by the
basicinterface_subclasses fixture.
[analyticalproblems] make BurgersProblem Unpicklable
[tests] rename basicinterface_subclasses -> basicinterface_subclass

pytest fixtures are usually given singular names as they appear as
argument names of the test methods.