Fix portability issue in tests from commit ce773f230.

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

Commit: 08b96a2b5243957acd3f7c3105af8b5b2d0da3ef
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2021-09-03T14:01:02Z
Releases: 14.0
Fix portability issue in tests from commit ce773f230.

Modern POSIX seems to require strtod() to accept "-NaN", but there's
nothing about NaN in SUSv2, and some of our oldest buildfarm members
don't like it.  Let's try writing it as -'NaN' instead; that seems
to produce the same result, at least on Intel hardware.

Per buildfarm.

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