[libxml] Unit test for libxml_disable_entity_loader()

Summary:
Test to make sure libxml_disable_entity_loader() is behaving.
Merge branch 'PHP-5.4' into PHP-5.5

* PHP-5.4:
  [libxml] Unit test for libxml_disable_entity_loader()
Merge branch 'PHP-5.5' into PHP-5.6

* PHP-5.5:
  [libxml] Unit test for libxml_disable_entity_loader()
Merge branch 'PHP-5.6'

* PHP-5.6:
  [libxml] Unit test for libxml_disable_entity_loader()