Harden pg_filenode_relation test against concurrent DROP TABLE.

Noah Misch <noah@leadboat.com>

Commit: cc3e1553fa6786b3a8c673fc9d8ce0a258ba8809
Author: Noah Misch <noah@leadboat.com>
Date: 2014-06-13T23:58:58Z
Releases: 9.4.0
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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