Pause after assigning Listen in tests to ensure node is handling request

Test flakiness is seen, from time to time, in tests involving Listen
requests. Debugging has shown that the flakiness is due to the node not
having started handling the Listen request when the client is sending
records which it expects to receive back via the Listen. Simply pausing
after assigning the Listen request greatly decreases this flakiness. A real
fix is not possible, without changing the legacy Listen protocol, which
isn't going to happen.

src/dhttest/DhtClient.d

@@ -35,6 +35,7 @@ class DhtClient
     import ocean.core.Array : copy;
     import ocean.task.Task;
     import ocean.task.Scheduler;
+    import ocean.task.util.Timer : wait;
 
     import swarm.client.plugins.ScopeRequests;
     import swarm.util.Hash;
@@ -556,6 +557,16 @@ class DhtClient
         this.swarm_client.assign(this.swarm_client.listen(channel,
             &listener.record, &listener.notifier));
 
+        // FLAKY: As there is no notification when the Listen request has
+        // started being handled by the node, it's possible that the dummy
+        // record put (see below) will arrive before the Listen request is
+        // registered with the channel to receive updates. Given the lack of
+        // notification, it's impossible to write a non-flaky test for this.
+        // In this circumstance, simply waiting 100ms after assigning the Listen
+        // request -- while it could, in principle, still fail -- will massively
+        // reduce flakiness.
+        wait(100_000);
+
         auto original_value = this.get(channel, key);
         this.put(channel, key, "dummy_value"[]);