Currently, there are absolutely no tests for this project.
This is/was, in part, due to time constraints and the need to get a stable release out the door.
It is also in part due to the requirements of external libraries and PECL extensions that will make setting up DrupalCI... challenging (the latter not, yet, supported #2949210: Drupalci Roadmap → ).
We can, however, start to implement tests for the Drupal side of things (i.e. plugin managers, plugins, etc.).
Split out areas where tests are needed and create sub-tasks for implementing them.
Needs work
3.0
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
I've written a few tests to render a text with each markdown parser. Only for the CommonMark PECL parser I marked the test as incomplete because that one requires a PECL extension. Without the markTestIncomplete() call, the fails with the error Call to undefined function CommonMark\Parse().