Improve stability of recently-added regression test case.

Improve stability of recently-added regression test case.

Commit b5febc1d1 added a contrib/btree_gist test case that has been
observed to fail in the buildfarm as a result of background auto-analyze
updating stats and changing the selected plan.  Forestall that by
forcibly analyzing in foreground, instead.  The new plan choice is
just as good for our purposes, since we really only care that an
index-only plan does not get selected.

Back-patch to 9.5, like the previous patch.

Discussion: https://postgr.es/m/14643.1539629304@sss.pgh.pa.us