Harden pg_filenode_relation test against concurrent DROP TABLE.

Noah Misch <noah@leadboat.com>

Commit: f3fdd257a430ff581090740570af9f266bb893e3
Author: Noah Misch <noah@leadboat.com>
Date: 2014-06-13T23:57:59Z
Releases: 9.5.0
Harden pg_filenode_relation test against concurrent DROP TABLE.

Per buildfarm member prairiedog.  Back-patch to 9.4, where the test was
introduced.

Reviewed by Tom Lane.

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