includejs test has this code:

'<script src="' . $dir . '/templates/parent_and_child.html?assets_version=1"></script>',
'<script src="' . $dir . '/parent_templates/parent_only.html?assets_version=1"></script>',
'<script src="' . $dir . '/templates/child_only.html?assets_version=1"></script>'

The tag is intended for javascript. I suggest changing the test to include js files.

Also $dir is absolute directory. Template tests should use relative directories for templates, just like phpBB does.