C: incr CV and GP->FORM refcounts against glob_assign_glob

CVs on 5.10 are always refcnt+1 because they cannot be deleted properly.
esp. for the body in sv_clear:
else if (sv_type_details->body_size) {
    safefree(SvANY(sv));

we need the same for GP->FORM fields
in the case of glob_assign_glob() which does gp_free.

testcase: t/CORE/op/gv.t:171 *x = *STDOUT;
fixes #294
MANIFEST: renamed coretests