For rationale, see https://github.com/php/php-src/pull/6787
Make extension checks lowercase, add a special case for opcache
that has internal name not matching .so filename.
Extensions migrated in part 2:
* dom
* exif
* fileinfo
* ffi
They cover
- The basic behaviour
- When the method receives as argument a file which doesn't exist
- When the argument is an empty string
- When an empty file is loaded
- When a not-well formed html file is loaded
It also includes 2 auxilary files
- An empty html file
- A not well formed html file
