Fix setUpModule and tearDownModule

Currently the test runner does not respect https://docs.python.org/3/library/unittest.html#setupmodule-and-teardownmodule, this needs fixing.

This is somewhat related to #51 , as we should probably reuse Python's test loader / runner, and hook TestSlide into them, so we can avoid similar potential issues in the future.

This will likely require implementing `@context.before_once` and `@context.after_once` at TestSlide's DSL.