Migrate SKIPIF -> EXTENSIONS (#7138)

This is an automated migration of most SKIPIF extension_loaded checks.

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.

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