Improve regression test coverage for hash indexes.

Improve regression test coverage for hash indexes.

On my system, this improves coverage for src/backend/access/hash from
61.3% of lines to 88.2% of lines, and from 83.5% of functions to 97.5%
of functions, which is pretty good for 36 lines of tests.

Mithun Cy, reviewing by Amit Kapila and Álvaro Herrera