regularise test for Carp vivifying B stash

There's an old test to check that Carp doesn't vivify B:: despite
attempting to use B::svref_2object().  Turn this into the same kind of
test now used for overload::StrVal() and utf8::downgrade(), testing more
cleanly for stash vivification and also testing for GV vivification.
avoid package name "B" in Carp tests

[rt.cpan.org #76167]  For tests not relating to the B module, avoid using
"B" as a package name.