TST: tests for random variables

[eigenfoo]

I was just thinking about the test suite here - I think it makes sense to dynamically generate tests for each random variable to:

1. Instantiate it
2. .sample from it
3. Evaluate its .log_prob
4. Convert it as_tensor

In other words, we should try to pull a similar trick to how we dynamically wrap the tfp distributions as random variables, for the tests. I think has several advantages, (e.g. the tests write themselves!), but I'm unsure if this is best practice with tests... Any thoughts?

cc @canyon289 depending on how far you want to take #49, I'd be happy to do a follow up PR for tests.