Temporarily install debugging in partition_prune test

Temporarily install debugging in partition_prune test

The buildfarm animal parula has been sporadically failing in the
partition_prune test for the past week or so.  It appears like an
auto-vacuum or auto-analyze has run on one of the partitions of the "ab"
table, causing the plan to change.  This is unexpected as the table is
empty.

Here we install some telemetry to find out if this is the case.  This
also joins in pg_index to see if something has gone wrong with the index
which could result in the planner being unable to use that index.

We can revert this once we've figured out the cause of the plan
instability.

Reported-by: Tom Lane
Investigation-by: Tom Lane
Discussion: https://postgr.es/m/4009739.1710878318%40sss.pgh.pa.us