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