Core Test Cleanups

### 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