Add unit tests for Twig tag

Problem/Motivation

This module adds a {% theme %} Twig tag and we don't have any tests. That makes it difficult to ensure we don't break stuff.

Proposed resolution

Add tests.

Remaining tasks

Write them. https://www.drupal.org/docs/8/phpunit

User interface changes

None.

API changes

NONE. And, after the tests are written, we'll know when API changes actually happen.