Stabilize postgres_fdw test

Stabilize postgres_fdw test

The test fails when RESET statement_timeout takes longer than 10ms.
Avoid the problem by using SET LOCAL instead.

Overall, this test is not ideal: 10ms could be shorter than the time to
have sent the query to the "remote" server, so it's possible that on
some machines this test doesn't actually witness a remote query being
cancelled.  We may want to improve on this someday by using some other
testing technique, but for now it's better than nothing.  I verified
manually that one round of remote cancellation occurs when this runs on
my machine.

Discussion: https://postgr.es/m/CAGECzQRsdWnj=YaaPCnA8d7E1AdbxRPBYmyBQRMPUijR2MpM_w@mail.gmail.com