pgsql: 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.

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9ddd1c27ee27e6d06349e4dc84baf14333d23311

Modified Files
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contrib/seg/expected/seg.out | 2 ++
contrib/seg/sql/seg.sql | 2 ++
2 files changed, 4 insertions(+)