New unit tests

Switch the unit tests to use the global WP_TESTS_DIR install of the wp test framework, as scaffolded by wp-cli.

See the updated readme in https://github.com/humanmade/backupwordpress-dev/commit/2312074cd1cd6e92d84da2d0ebde62a00f3c8395 for details of how to setup and run the unit tests under this new system.