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

Author: RhodiumToad