Migrate skip checks to --EXTENSIONS--, p3

For rationale, see #6787

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

Always print numeric opline numbers

Reindent phpt files

Convert ZEND_ECHO operand to string after sccp

And filter out echoes of the empty string (e.g. false/null)

Split out of #5097 (on GitHub)

Closes GH-5118