pgsql: Stabilize new GIN test case in 9.5 branch.

Stabilize new GIN test case in 9.5 branch.

In 9.6 and up, gin_test_tbl has autovacuum_enabled = off thanks to
commit f8a1c1d5a. 9.5 lacked that, which allowed autovacuum to
bollix the results of the test case added by commit 8150f7813.
We could fool with disabling seqscan around that test, but making
this branch look more like the later ones seems a better answer.

Per buildfarm member protosciurus. (I'm not very sure why
protosciurus is the only animal to report this so far; but it'd
clearly be a timing-related failure, so it's not astonishing that
only some machines would show it.)

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e82b6e00ac60d0716161352d448660c014e092a7

Modified Files
--------------
src/test/regress/expected/gin.out | 2 +-
src/test/regress/sql/gin.sql | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)