Clean up testsuite

[ksimpson-work]

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

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[ksimpson-work]

/ok to test

[leofang]

Thanks, Keenan, left some nits.

[ksimpson-work]

Thanks @leofang for those comments. I addressed all of them and made the changes.

[leofang]

Thanks, Keenan! Left 3 nits, feel free to merge after addressing them.