fresh_perl.t: Make the test for #3066 more explicit

This test was added to make sure that constants don’t become undefined
as a result of being shared between ops.

What was tested, though, was a side-effect, and not the actual bug
itself.

This behaviour has changed (sub(){42} now returns a mutable val-
ue), so this test needs to change, too.  It was only passing under
ithreads, and only as the result of another bug, which the next com-
mit will fix.
Stop op freeing from interfering with sub(){42} mutability

The problem is that when an OP_CONST is freed, it calls pad_swipe,
which clears the PADTMP flag when removing the SV from the pad.  Since
PADTMP no longer necessarily means ‘in a pad’, we shouldn’t turn
this flag off.
Increase $XS::APItest::VERSION to 0.55