Migrate skip checks to --EXTENSIONS--, p3

For rationale, see #6787

Extensions migrated in part 3:
* ftp
* gmp
* iconv
* opcache
* shmop

Add many missing closing PHP tags to tests

Closes GH-5958

Implement match expression

RFC: https://wiki.php.net/rfc/match_expression_v2

Closes GH-5371.