Automatic plugin CI testing

[HongPong]

Improving testing on the plugin would make all development more stable and reduce chance of regressions. Note this would require a site populated with content that has both valid and 404 outgoing links.

In this I am inspired by the testing code on Linkback for Drupal 8 which is very detailed for all functions.

• https://make.wordpress.org/cli/handbook/plugin-unit-tests/
• https://torquemag.io/2018/06/advanced-oop-for-wordpress-part-6-continuous-integration-for-wordpress-plugins/
• https://carlalexander.ca/continuous-integration-wordpress/
• https://www.smashingmagazine.com/2017/12/automated-testing-wordpress-plugins-phpunit/
• https://generalchicken.net/wp-codeception-tutorial/your-first-cept-acceptance-test/