Add php tests

[titaniumbones]

From @titaniumbones on November 11, 2017 7:51

need to add phpunit to the docker image, and at least some php tests to the theme.

Copied from original issue: titaniumbones/wordpress-starter#14

[titaniumbones]

https://github.com/pixline/wp-cli-theme-test-command
https://github.com/Evinceknow/Theme-Unit-Test-Environment-for-WordPress
https://github.com/INN/Largo-Sample-Child-Theme/tree/master/tests
https://mikevanwinkle.com/wordpress-unit-testing-travis-ci-awesome/
https://make.wordpress.org/cli/handbook/plugin-unit-tests/
https://www.kirstencassidy.com/testing-wordpress-plugins-wp-cli-phpunit/