Clean up the testsuite by:

1. Removing calls to internal entry-points in favour of public ones.
2. Updating the use of fixtures to handle context initialization on a per-test basis. This includes a function level fixture to init, and then subsequently de-init when calling a method which requirts context initalization (usually via device.set_current()). It also includes a fixture to deinit after some tests which call set_current() as part of their logic.
3. As an effect of moving to public entry-points, the tests become more relational. The use of examples as additional tests also increases relational test coverage'

closes #196