Use custom hash opclass for hash partition pruning

Use custom hash opclass for hash partition pruning

This custom opclass was already in use in other tests -- defined
independently in every such file.  Move the definition to the earliest
test that uses it, and keep it around so that later tests can reuse it.
Use it in the tests for pruning of hash partitioning, and since this
makes the second expected file unnecessary, put those tests back in
partition_prune.sql whence they sprang.

Author: Amit Langote
Discussion: https://postgr.es/m/CA%2BTgmoZ0D5kJbt8eKXtvVdvTcGGWn6ehWCRSZbWytD-uzH92mQ%40mail.gmail.com