Core Test Cleanups

[kclowes]

What was wrong?

There are some test cleanups that we'd like to look into.

How can it be fixed?

Some ideas which should probably be separate PRs if implemented:

• explore the possibility of sharing contracts between async_w3 and w3.
• explore skip_if_testrpc. Decide if they add enough documentation to keep around, or if documentation should be moved to the docs.
• explore which tests (if any) can be removed from the async core tests
• split async core tests into a separate tox run
• some of the emitter fixtures (and maybe others?) are duplicated throughout the tests, explore correcting the scope of the fixtures