Speed up hash_index regression test.

Commit f5ab0a14 made this test take substantially longer than it used
to.  With a bit more care, we can get the runtime back down while
achieving the same, or even a bit better, code coverage.

Mithun Cy

Discussion: https://postgr.es/m/CAD__Ouh-qaEb+rD7Uy-4g3xQYOrhPzHs-a_TrFAjiQ5azAW5+w@mail.gmail.com