Fix unstable regression test added by commits 59b71c6fe et al.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: fa39617a64d4134c63b3f62ae12b0268542a79e5
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2017-11-24T05:29:20Z
Releases: 9.6.7
Fix unstable regression test added by commits 59b71c6fe et al.

The query didn't really have a preferred index, leading to platform-
specific choices of which one to use.  Adjust it to make sure tenk1_hundred
is always chosen.

Per buildfarm.

Files