Migrate skip checks to --EXTENSIONS--, p2

For rationale, see https://github.com/php/php-src/pull/6787

Make extension checks lowercase, add a special case for opcache
that has in

Migrate skip checks to --EXTENSIONS--, p2

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

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Fix #79271: DOMDocumentType::$childNodes is NULL

Dom level 2 core, DOM level 3 core and the DOM living standard agree
that `childNodes` always return a `NodeList`, and never `null`.