Move all testing code into the main library

[neerajprad]

As pointed out by @fritzo, any code in the test folder will be unusable by other projects, so it makes sense to move these to testing instead. Refer to pyro-ppl/pyro#2053 for details.

[fritzo]

I've been thinking about this. I think it should be possible to register test cases in pytest-oblivious maner by creating an object with

• a function (the name of this function will be used as test case)
• a grid of test parameters that we can somehow thread to pytest.mark.parametrize

Again, it will help us design this interface to have more concrete tests to generalize from, e.g. MCMC, SVI, data generation, prediction.