Have a script to generate a list of slowest test fixtures

[veox]

Fallout from issue #1857, in particular this comment.

What is wrong?

Keeping track of slowest fixtures from ethereum/tests, and regenerating their list, has previously been done manually.

How can it be fixed

In issue #1857, I've linked "napkin-quality scripts" that were used to do this for Istanbul. See if that can be incorporated in the repo.