Migrate skip checks to --EXTENSIONS--, p3

For rationale, see #6787

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

Fix arg/func info

* mysqli_get_server_info() cannot return null. The underlying API
is infallible.
* mysqli_select_db() func info is redundant.
* mb_detect_order() can only

Fix arg/func info

* mysqli_get_server_info() cannot return null. The underlying API
is infallible.
* mysqli_select_db() func info is redundant.
* mb_detect_order() can only return array|true, not array|false.

Also make the func_info.phpt test that is supposed to catch these
kinds of issues actually work.

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Add tests to check mismatching function signatures

Closes GH-5666