Add zend_test.observer.enabled=0 to opcache tests asserting specific TMP count

Necessary to succeed when tests are run with zend_test.observer.enabled=1.

Migrate skip checks to --EXTENSIONS--, p3

For rationale, see #6787

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

Optimize out no-op `yield from` statements

If the array is empty, then I'd expect that the generator is never left,
and that can be converted to a no-op and the return value would always

Optimize out no-op `yield from` statements

If the array is empty, then I'd expect that the generator is never left,
and that can be converted to a no-op and the return value would always be `null`.

Make `yield from [];` as efficient as `if (false) { yield null; }`
when opcache's sccp pass is enabled.

Closes GH-5679

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