Partial backport of e7462bf

e7462bff19228162a21ccc6dd248b0dbffc15517

Run one testsuite with observers enabled in CI (#13869)

Signed-off-by: Bob Weinand <bobwei9@hotmail.com>

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.

Add zend_observer API

Closes GH-5857.

Co-authored-by: Nikita Popov <nikita.ppv@gmail.com>
Co-authored-by: Sammy Powers <sammyk@datadoghq.com>