Add/Implement e2e tests

[ecgan]

Continuing from #38, I believe it's worth the effort to implement end-to-end testing too.

I'm using Gutenberg's end-to-end testing as reference material. I am using @wordpress/env locally right now, will need to write some tests to make sure if it really works well.

[jconroy]

Just a heads up that Woo also has its own setup for E2E https://github.com/woocommerce/woocommerce/tree/master/tests/e2e/env which is aimed at being used as much as possible across the division for consistency

An internal post discusing its use in AutomateWoo is located here pb0Spc-S6-p2 - it really just steps through @nima-karimi experience and communication in trying to get it set up.