Tests: give the fail2ban and task-manager checks room under load

Two long-standing flakes in system_tests, both timing:

- security: the ban period was set to one second, but the test makes a
  hundred more requests before it checks that a valid password from the
  banned address is refused, so under load the ban had already expired.
  Five seconds, and the expiry check sleeps six.
- task: a task scheduled one second out was queried for straight away,
  and under load the query landed after the task manager had run and
  removed it. Three seconds.

Seven consecutive system_tests runs, neither recurred.

tests/src/system/task.rs

@@ -39,7 +39,9 @@ pub async fn test(test: &mut TestServer) {
     admin.assert_no_tasks().await;
 
     // Create a successful task for future execution
-    admin.schedule_test_task(TASK_SUCCESS, 1).await;
+    // inbuxa: three seconds, not one: the query below takes longer than a
+    // second under load, and the task had already run and been removed
+    admin.schedule_test_task(TASK_SUCCESS, 3).await;
     admin.assert_has_tasks(1).await;
     admin.assert_no_tasks().await;