* Don't merge test options.
Particularly, I don't want `lodash/merge` to merge my specific plugins
with the general test plugins. It led to odd behavior where I could
enable a loose transform in my specific test, just to have it overridden
by the test fixture's general options.
* Need options
* Handle arrow function processing via shared API rather than default plugin.
* Fix a few small PR comments.
* Preserve existing spec arrow 'this' rewrites, and support spec in subclass constructors.
