Add tests for non-mock sample generation

[tdiethe]

Currently the tests of sampling from distributions all use the mock test generator, which doesn't actually test the random_gen.py functions, with the exception of the Bernoulli test:

        # Also make sure the non-mock sampler works
        rand_gen = None
        var = Bernoulli.define_variable(0, shape=rv_shape, rand_gen=rand_gen, dtype=dtype).factor
        prob_true_mx = mx.nd.array(prob_true, dtype=dtype)
        if not prob_true_is_samples:
            prob_true_mx = add_sample_dimension(mx.nd, prob_true_mx)
        variables = {var.prob_true.uuid: prob_true_mx}
        rv_samples_rt = var.draw_samples(
            F=mx.nd, variables=variables, num_samples=num_samples)

        assert is_sampled_array(mx.nd, rv_samples_rt)
        assert get_num_samples(mx.nd, rv_samples_rt) == num_samples
        assert rv_samples_rt.dtype == dtype