Migrate SKIPIF -> EXTENSIONS (#7138)

This is an automated migration of most SKIPIF extension_loaded checks.

Partially revert "Adapt test cases for Oniguruma 6.9.4"

This partially reverts commit c55d09c2f547634b577aa5aeaa1438d772bc29d1,
because `MB_ONIGURUMA_VERSION` is only available as of PHP

Partially revert "Adapt test cases for Oniguruma 6.9.4"

This partially reverts commit c55d09c2f547634b577aa5aeaa1438d772bc29d1,
because `MB_ONIGURUMA_VERSION` is only available as of PHP 7.4.0, so
that change made no sense for PHP-7.3; we keep it for PHP-7.4, though.
We also stick with the modification to bug78633.phpt.

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Adapt test cases for Oniguruma 6.9.4

Apparently, bug 78633 has now really been fixed; the former fix only
catered to the buffer overflow, but yielded a wrong result. Also,
the order

Adapt test cases for Oniguruma 6.9.4

Apparently, bug 78633 has now really been fixed; the former fix only
catered to the buffer overflow, but yielded a wrong result. Also,
the order of the named captures has been fixed.

show more ...