Previously this relied on getting all known classes/functions/etc
when it was loaded, and then again after the PHP module being
tested was loaded. This approach no longer works now we've
stopped loading modules using dl(), so use ReflectionExtension
instead to get information about a specific extension.
This is likely also faster than wading through lists including
everything predefined by PHP.
Without inventing a SWIG/PHP-specific mechanism, we can't really
finalise objects in the way the testcase expects, so adjust the
testcase minimally so we avoid triggering C++ undefined behaviour
(use-after-free).
