Fix outdated code reference
ExecCreatePartitionPruneState was renamed by commit
297daa9d4353, but
this test file didn't get the memo. Repair.
Author: Amit Langote
Discussion: https://postgr.es/m/CA+HiwqFLw=oLX0tP9kcKBmoOExNjDaoAe99dRcxo-GdB9abP9A@mail.gmail.com