pgsql: pg_test_timing: fix unit in backward-clock warning

pg_test_timing: fix unit in backward-clock warning

pg_test_timing reports timing differences in nanoseconds in master, and
in microseconds in v14 through v18, but previously the backward-clock
warning incorrectly labeled the value as milliseconds.

This commit fixes the warning message to use "ns" in master and
"us" in v14 through v18, matching the actual unit being reported.

Backpatch to all supported versions.

Author: Chao Li <lic(at)highgo(dot)com>
Reviewed-by: Lukas Fittl <lukas(at)fittl(dot)com>
Reviewed-by: Xiaopeng Wang <wxp_728(at)163(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/F780CEEB-A237-4302-9F55-60E9D8B6533D@gmail.com
Backpatch-through: 14

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/fb5acf2d8f2eb2a91f59a2e790a23d1c45d9de19

Modified Files
--------------
src/bin/pg_test_timing/pg_test_timing.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)