Migrate SKIPIF -> EXTENSIONS (#7138)

This is an automated migration of most SKIPIF extension_loaded checks.

Rename zend-test to zend_test

The extension name should match the name of the ext/ directory,
otherwise it will not get picked up by run-tests. It would be possible
to remap this in

Rename zend-test to zend_test

The extension name should match the name of the ext/ directory,
otherwise it will not get picked up by run-tests. It would be possible
to remap this in run-tests, but I think it's better to rename the
extension to follow the standard format. Other extensions also
use underscore instead of hyphen (e.g. pdo_mysql and not pdo-mysql).
Of course, the ./configure option remains hyphenated.

Closes GH-6613.

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Fix observer tests on Windows

Use %e instead of a hardcoded forward slash.

Disable opcache optimizations during some observer tests

Opcache inlines functions that only return a constant. Disable
optimizations to prevent differences in tests where such functions

Disable opcache optimizations during some observer tests

Opcache inlines functions that only return a constant. Disable
optimizations to prevent differences in tests where such functions
are used (or rewrite the test to not depend on it).

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Provide unused retvals to observers

Make sure that the return value is available to observers, even if
it is not used by the caller.

Closes GH-6422.