testsuite: update purge tests for job-list changes

Problem: The job-list module now stores / recovers inactive job
data from its internal database.  Tests that assume job-list
only stores jobs in memory do not account for this, leading to
errors.

Solution: Update tests to read the job-list module stats instead
of using `flux jobs`.  This gathers internal data to the job-list
module, which allows purge tests to pass.

t/t2809-job-purge.t

@@ -13,7 +13,7 @@ inactive_count() {
 	if test $how = "job-manager"; then
 		flux module stats --parse=inactive_jobs job-manager
 	elif test $how = "job-list"; then
-		flux jobs --no-header --filter=inactive|wc -l
+		flux module stats -p jobs.inactive job-list
 	elif test $how = "job-list-stats"; then
 		flux job stats | jq .job_states.inactive
 	else