Domains: Move all lib/domains tests to local single test runner

[gziolo]

Part of #3822.

This is similar to #3773. It does exactly the same steps to speed up test execution.

This pull request seeks to eliminate subtree-specific Makefiles in the client/lib/domains directory, used always for testing, with a single test runner for the entirety of the client/lib/domains directory.

The changes eliminated 4 Makefiles.

Benefits include:

• Improved performance by decreasing number of Mocha/Babel reinitializations
• Fewer files to maintain

Testing instructions:

Ensure tests pass by running make test from the project root directory.

[gziolo]

/cc @aduth @gwwar @blowery @mtias

[gwwar]

👍 Maybe add a quick usage note in client/lib/domains/README.md and

[blowery]

Maybe we move this to lib and let things start opting in there instead? too big?

[blowery]

Nah, ship it. Can address later.

[gziolo]

@gwwar: Usage note added with 7881dee

@blowery: I'm still not sure how to tackle it at a global level. Definitely it's going to be easier to split all work needed if we start grouping tests in subfolder that aren't on the very top level. We can always move that level up :)

Thanks for a review!