pgsql: Expand coverage of parallel gather merge a bit.

Expand coverage of parallel gather merge a bit.

Previously paths reaching heap_compare_slots weren't covered.

Author: Rushabh Lathia
Reviewed-By: Andres Freund
Discussion:
/messages/by-id/CAGPqQf3C+3PBujb+7m=ceWeii4-vBY=XS99LjzrpkpefvzJbFg@mail.gmail.com
/messages/by-id/27200.1502482851@sss.pgh.pa.us
Backpatch: 10, where gather merge was introduced

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ee5572e051955ef4570f95413dc6cddb4df51d50

Modified Files
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src/test/regress/expected/select_parallel.out | 69 +++++++++++++++++++++------
src/test/regress/sql/select_parallel.sql | 14 ++++--
2 files changed, 65 insertions(+), 18 deletions(-)

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