Introduce unit tests via the WordPress Core test framework

[stevegrunwell]

It's not much, but it's a start: using WP-CLI's wp scaffold plugin-tests command, I've introduced unit tests to the project, starting with the batcache_post() function in batcache.php.

As with most tests, the goal will be to prevent regressions and make it easier to introduce new functionality without breaking the internet (as that's typically frowned upon).

[aidvu]

This is super nice. Would you mind revisiting and checking if they run with latest WP/PHP 7.x. I'd love to build on this work.