pgsql: Fix unstable partition_prune regression tests

Fix unstable partition_prune regression tests

This was broken recently by a929e17e5.  I'd failed to remember that
parallel tests should have their EXPLAIN output run through the
explain_parallel_append function so that the output is stable when
parallel workers fail to start.

fairywren was first to notice.

Reported-by: Michael Paquier
Discussion: https://postgr.es/m/20201102062951.GB15770@paquier.xyz

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/90d8f1b1826ce076a502a43fe7c88423b46c6349

Modified Files
--------------
src/test/regress/expected/partition_prune.out | 45 +++++++++++++--------------
src/test/regress/sql/partition_prune.sql      |  9 +++---
2 files changed, 26 insertions(+), 28 deletions(-)