Add tests for class AMP_Core_Theme_Sanitizer

[schlessera]

Feature description

Although we are testing the entire matrix of PHP versions through Travis CI, release 1.4 introduced code that worked on PHP 7.2+ only as part of AMP_Core_Theme_Sanitizer::add_twentytwenty_modals, see #3727.

This shouldn't happen, and it was only possible because we don't have unit tests in place to cover the code in the AMP_Core_Theme_Sanitizer class.

Acceptance criteria

Unit tests cover each of the methods in the code of the AMP_Core_Theme_Sanitizer class, at the very least for the happy path.

Implementation brief

QA testing instructions

Demo

Changelog entry