Use test-fixture fork-selection helpers from `py-evm` when they become available

[veox]

• Trinity version: should apply to master still if this issue not outdated
• OS: IRRELEVANT
• Python version (output of python --version): IRRELEVANT
• Environment (output of pip freeze): IRRELEVANT

What is wrong?

(Fallout from PR #608.)

Parts of the test configuration are copy-paste from py-evm, after the py-evm/trinity repo split:

• the tests that xfail in the pinned version of py-evm will also xfail in trinity, but ATM they can't be imported, so the list is copy-pasted manually;
• similar for "human comfort" pytest flag --fork;
• possibly other stuff?..

In an interim commit of PR #608, these were marked with TODOs:

• https://github.com/ethereum/trinity/pull/608/files#diff-08fccd373dc59fca958451434197a951R107
• https://github.com/ethereum/trinity/pull/608/files#diff-08fccd373dc59fca958451434197a951R213

Also tracked in py-evm as issue ethereum/py-evm#1781.

How can it be fixed

🖖