Change default method of disabling JIT

https://wiki.php.net/rfc/jit_config_defaults
Closes GH-12678

Improve jit tests

Closes GH-12425

Migrate skip checks to --EXTENSIONS--, p3

For rationale, see #6787

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

Fixed bug #79888 (Incorrect execution with JIT enabled)