Clean up test_cloexec.c and Makefile.

Clean up test_cloexec.c and Makefile.

An unused variable caused a compiler warning on BF animal fairywren, an
snprintf() call was redundant, and some buffer sizes were inconsistent.
Per code review from Tom Lane.

The Makefile's test ifeq ($(PORTNAME), win32) never succeeded due to a
circularity, so only Meson builds were actually compiling the new test
code, partially explaining why CI didn't tell us about the warning
sooner (the other problem being that CompilerWarnings only makes
world-bin, a problem for another commit).  Simplify.

Backpatch-through: 16, like commit c507ba55
Author: Bryan Green <dbryan.green@gmail.com>
Co-authored-by: Thomas Munro <tmunro@gmail.com>
Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/1086088.1765593851%40sss.pgh.pa.us