testsuite: cover job stats update

Problem: there is no coverage to ensure flux-job --stats-only
shows correct inactive job count after purge.

Add a test.

t/t2809-job-purge.t

@@ -14,8 +14,13 @@ inactive_count() {
 
 	if test $how = "job-manager"; then
 		flux module stats --parse=inactive_jobs job-manager
-	else # job-list
+	elif test $how = "job-list"; then
 		flux jobs --suppress-header --filter=inactive|wc -l
+	elif test $how = "job-list-stats"; then
+		flux job stats | jq .job_states.inactive
+	else
+		echo Unknown method: $how >&2
+		return 1
 	fi
 }
 
@@ -137,6 +142,9 @@ test_expect_success 'reconfigure job manager with inactive-age-limit=1ms' '
 test_expect_success 'wait for job-list inactive job count to reach 0' '
 	wait_inactive_count job-list 0 30
 '
+test_expect_success HAVE_JQ 'confirm job-list stats show zero inactive jobs' '
+	test $(inactive_count job-list-stats) -eq 0
+'
 test_expect_success 'reconfigure job manager with incorrect type limit' '
 	cat >config/system.toml <<-EOT &&
 	[job-manager]