systematic tests for sampling methods

[janfb]

At the moment it seems we do not have a systematic suite of tests for the growing number of sampling methods. Or, they are scattered in different test modules, e.g., the mcmc tests in mcmc_test and the sir tests in vi_test.

I think it would be great to have one joined samplers_test.py collecting them all.

To complete this issue, we should:

• [ ] create a new test file samples_test.py
• [ ] define desired tests for mcmc samplers in it (TODO: need to figure out what exactly we want.
• [ ] remove tests that are then duplicates across mcmc_test, sir, vi_test
• [ ] Time the tests and ensure that they are efficient

[janfb]

related to #908: we can add centralized tests when introducing new MCMC samplers.