testsuite: don't expect debug.free-request event

Problem: free requests may not take place in the context of the
job once housekeeping is in place, but t2212-job-manager-plugins.t
uses the debug.free-request event as an indication that the job
debug flag could be set.

Use debug.alloc-request in the test instead.

t/t2212-job-manager-plugins.t

@@ -253,7 +253,7 @@ test_expect_success 'job-manager: load jobtap_api test plugin' '
 test_expect_success 'job-manager: test that job flags can be set' '
 	id=$(flux submit \
 	       --setattr=system.depend.set_flag=debug hostname) &&
-	flux job wait-event -vt 20 $id debug.free-request &&
+	flux job wait-event -vt 20 $id debug.alloc-request &&
 	flux job wait-event -vt 20 $id clean
 '