Breakup tests into separate test files

Not sure if this you're interested in this change, but I was working with some colleagues on testing Stripe and we noticed that all the tests were specified in the `test_stripe.rb`. This made it a little harder to go right to where we wanted to test from the code that we were changing. If not a change you're interested in, it's not a big deal, we can go back to tests in the one large file.