Convert test suite to use declarative tool

[pipermerriam]

What was wrong?

#140 introduced a declarative way to write implementation agnostic tests.

https://lahja.readthedocs.io/en/latest/api/testing.html

Some of the test suite has been converted to use this but not all of it.

How can it be fixed?

Take some or all of the remaining tests that are under ./tests/core/asyncio or ./tests/core/trio and rewrite them using the declarative tool. New tests should live under ./tests/core/common