Fixed issue with HTML tags in custom head scripts

Fixed issue with HTML tags in custom head scripts

Fixes a strange issue of HTML tags within script tags being malformed
when part of the HTML custom head content due to the PHP parsing we do.
DOMDocument seemed to cause this upon load.
Adding LIBXML_SCHEMA_CREATE to the ->loadHTML call seems to fix this but
not really sure why. Doesn't seem to cause further issues though.
Tested with multiple scripts and styles and comments and meta tags.

- Also added new testing class to cover.
- As part of testing, added new folder within tests to house setting
  specific tests.

For #2914