Improve class constant fetch fix

Dereference the cached constant for Test::TEST as well (and not just
self::TEST).

Also improve the phpt file to test this case as well - previously
this only manifested with opcache enabled, due to literal sharing.

Additionally the Z_TYPE_P != IS_REFERENCE assertion is now moved
into the TMP_VAR fetching code (as it applies to more than just
property assignments.)