Band-aid new postgres_fdw test case to remove error text dependency.

Band-aid new postgres_fdw test case to remove error text dependency.

Buildfarm member lorikeet is still failing the test from commit
32a9c0bdf, but now it's down to the should-have-foreseen-it problem
that the error message isn't what the expected-output file expects.
Let's see if we can get stable results by printing just the SQLSTATE.
I believe we'll reliably see ERRCODE_CONNECTION_FAILURE, since
pgfdw_report_error() will report that for any libpq-originated error.

There may be a better way to do this, but I'd like to get the
buildfarm back to green before we discuss further improvements.

Discussion: https://postgr.es/m/E1kPc9v-0005L4-2l@gemulon.postgresql.org
Discussion: https://postgr.es/m/2621622.1602184554@sss.pgh.pa.us