Add regression test for two-phase transaction in postgres_fdw

Add regression test for two-phase transaction in postgres_fdw

postgres_fdw does not support two-phase transactions, so let's add a
small negative test case to check after it.  Note that this is checked
using an end-of-xact callback to ensure a proper connection cleanup with
the foreign server, which is called before checking if a server is able
to handle 2PC with max_prepared_xacts, so this test does not need an
alternate output file.

Author: Gilles Darold
Discussion: https://postgr.es/m/20191108090507.GC1768@paquier.xyz