Review and integrate relevant portions of the Vyper test fixture rewrite

[fubuloubu]

@jacqueswww just completed a rewrite of the Vyper test codebase. Check it out here: https://github.com/ethereum/vyper/blob/master/tests/conftest.py

Analyze his updates for ideas or modifications that would integrate well with this tool. Some functionality may not make sense for this.

For example, as he is testing a compiler, the fixtures he might have for writing test cases that involve raw code may not make sense for us to integrate into this tool. Think about how this can be enabled or integrated without creating additional complexity in this plugin or in a conftest.py fixture set for testing the compiler. (py-solc may need considerations as well)