Add marquise_shutdown() to the test gamut
Make a note to check this code later
Pretty sure we need to free out marquise_ctx as well

My C is rusty, but I believe we're correct to free the context, and we
don't expect to use it later.

To try and reassure myself that I wasn't doing anything drastically
wrong I ran valgrind against the python bindings with some test code. It
showed 16 bytes being definitely-leaked from marquise_init. This change
makes that go away. \o/