Stabilize contrib/seg regression test.

Stabilize contrib/seg regression test.

If autovacuum comes along just after we fill table test_seg with
some data, it will update the stats to the point where we prefer
a plain indexscan over a bitmap scan, breaking the expected
output (as well as the point of the test case).  To fix, just
force a bitmap scan to be chosen here.

This has evidently been wrong since commit de1d042f5.  It's not
clear why we just recently saw any buildfarm failures due to it;
but prairiedog has failed twice on this test in the past week.
Hence, backpatch to v11 where this test case came in.