pgsql: 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(dot)green(at)gmail(dot)com>
Co-authored-by: Thomas Munro <tmunro(at)gmail(dot)com>
Reported-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/1086088.1765593851%40sss.pgh.pa.us

Branch
------
REL_17_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/0451859131e530f3f5f906dc148d52011da48268

Modified Files
--------------
src/test/modules/test_cloexec/Makefile | 17 ++------
src/test/modules/test_cloexec/test_cloexec.c | 60 +++++++++-------------------
2 files changed, 23 insertions(+), 54 deletions(-)