don't rely on ghost contexts being unmolested

don't rely on ghost contexts being unmolested

Dying and returning from a format both relied on the state of a
just-popped context frame being preserved across a LEAVE.  Don't rely
on it.  Test using an operator ripped off from Scope::Cleanup, which makes
it easy to run arbitrary Perl code during cleanup, without isolating it
on a separate context stack as the DESTROY mechanism does.