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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Change Attribute Syntax from @@ to #[]

Add many missing closing PHP tags to tests

Closes GH-5958

Implement Shorter Attribute Syntax

RFC: https://wiki.php.net/rfc/shorter_attribute_syntax

Closes GH-5796.

Co-authored-by: Martin Schröder <m.schroeder2007@gmail.com>