Fix insufficiently-portable regression test case.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: c0f058e4d2c8dab6f6290dc85d2ad440691d562d
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2015-10-05T16:19:14Z
Releases: 9.5.0
Fix insufficiently-portable regression test case.

Some of the buildfarm members are evidently miserly enough of stack space
to pass the originally-committed form of this test.  Increase the
requirement 10X to hopefully ensure that it fails as-expected everywhere.

Security: CVE-2015-5289

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