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_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/a8dbe5288b0e9514713fc5de0a195e5f7d8d3fb1

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