Stabilize postgres_fdw tests on 32-bit machines

Stabilize postgres_fdw tests on 32-bit machines

cac169d68 adjusted DEFAULT_FDW_TUPLE_COST and that seems to have caused
a test to become unstable on 32-bit machines.

4b14e1871 tried to fix this as originally the plan was flipping between
a Nested Loop and Hash Join.  That commit forced the Nested Loop, but
there's still flexibility to push or not push the sort to the remote
server and 32-bit seems to prefer to push and on 64-bit, the costs
prefer not to.

Here let's just turn off enable_sort to significantly encourage the sort
to take place on the remote server.

Reported-by: Michael Paquier, Richard Guo
Discussion: https://postgr.es/m/ZUM2IhA8X2lrG50K@paquier.xyz