pgsql: Attempt to stabilize partition_prune test output.

Attempt to stabilize partition_prune test output.

Disable index-only scan for tests that might report variable results
for "Heap Fetches" statistic due to concurrent transactions affecting
whether all-visible flags can be set.

Author: David Rowley
Discussion: https://postgr.es/m/CAKJS1f_yjtHDJnDzx1uuR_3D7beDVAkNQfWJhRLA1gvPCzkAhg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b47a86f5008f2674af20dd00bc233e7b74e01bba

Modified Files
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src/test/regress/expected/partition_prune.out | 317 ++++++++++----------------
src/test/regress/sql/partition_prune.sql      |   7 +
2 files changed, 132 insertions(+), 192 deletions(-)