Fixed intermittent failure on AsyncMultiMapTest#testMapRemove

Cluster manager usually maintain caches of subscriptions.
When a node removes a subscription, other nodes get notified asynchronously and remove the entry from caches.

So we must not fail on first attempt to verify that ChooseableIterable is empty.

Signed-off-by: Thomas Segismont <tsegismont@gmail.com>

src/test/java/io/vertx/core/shareddata/AsyncMultiMapTest.java

@@ -111,11 +111,7 @@ public class AsyncMultiMapTest extends VertxTestBase {
                 map.remove("some-sub2", serverID3, onSuccess(res7 -> {
 
                   map.get("some-sub2", onSuccess(res8 -> {
-                    Set<ServerID> set3 = new HashSet<>();
-                    for (ServerID sid : res8) {
-                      set3.add(sid);
-                    }
-                    assertEquals(0, set3.size());
+                    waitUntil(res8::isEmpty);
                     testComplete();
                   }));
                 }));