Add tests

Some behaviour tests would be nice.

Tests to consider

• Check that each type of page returns correctly
  • Static pages
  • Static home page
  • Feed home page
  • Alternate feed page
  • Posts
  • Categories
  • Authors
  • Login page
  • Admin pages
  • REST API
• PHP scripts other than index.php and wp-admin/**/*.php return 404 and produce no unexpected output
• Regression #14 (closed): check that other URIs return 404
  • with end slash
  • without end slash
  • pagination type links (https://test.dev/pages/<page-slug>/<page-number>) to non-number values
• WP-CLI (for a number of different commands)
  • produces no warnings or errors on stderr
  • produces no unexpected output on stdout (logging, status)