devel/py-coverage: add a test target

[ngie]

Summary

This change adds a test target and relevant TEST_DEPENDS in order
to run the test suite via pytest.

Test Plan

Quite a few of the tests fail today; working on identifying the issues and resolving them (if possible).

[ngie]

coverage.py runs pytest via a separate script (igor.py). It's probably doing some extra setup/teardown that raw pytest isn't doing (I'm seeing a lot of ImportErrors with a module that exists in the tests directory).