[test] fix t/src/extend_vtable.t duplicate PMCNULL symbol (darwin)
[test] fix t/src/extend_vtable.t duplicate PMCNULL symbol (darwin)

fixes GH #1048.

youprofit/parrot,tkob/parrot,FROGGS/parrot,parrot/parrot,tkob/parrot,tkob/parrot,FROGGS/parrot,parrot/parrot,parrot/parrot,FROGGS/parrot,FROGGS/parrot,parrot/parrot,youprofit/parrot,youprofit/parrot,youprofit/parrot,FROGGS/parrot,FROGGS/parrot,tkob/parrot,youprofit/parrot,youprofit/parrot,youprofit/parrot,tkob/parrot,FROGGS/parrot,parrot/parrot,tkob/parrot,FROGGS/parrot,tkob/parrot,youprofit/parrot,tkob/parrot