pgsql: Ensure plan stability in contrib/btree_gist regression test.

Ensure plan stability in contrib/btree_gist regression test.

Buildfarm member skink failed with symptoms suggesting that an
auto-analyze had happened and changed the plan displayed for a
test query.  Although this is evidently of low probability,
regression tests that sometimes fail are no fun, so add commands
to force a bitmap scan to be chosen.

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/ab5f73b3ea3180959bb265811c6ac1d535c6f01c

Modified Files
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contrib/btree_gist/expected/not_equal.out | 2 ++
contrib/btree_gist/sql/not_equal.sql      | 4 ++++
2 files changed, 6 insertions(+)