Add tests for post-content manipulation

WordPress 5.2 changed how `get_the_content()` operates, and RCs revealed
the potential fragility of this plugin's approach.

Fixes #1.
3 jobs for fix/1-wp in 3 minutes and 12 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #3442
PHPunit:PHP7.2:MySQL

00:02:37

passed #3443
PHPunit:PHP7.3:MySQL

00:02:41

 
  Deploy
passed #3444
PluginSVN

00:00:31