sinttest: Use Thread.sleep(15) instead of Thread.yield()

Guus reports that the entity caps sinttest fails on openfire with an
timeout exception on Java 11. Very well possible that this is caused
by a changed scheduling behavior where the yield() thread nevertheless
dominates the, potential single, core.

The waitUntilThread() method is essentially a broken approach anyway
and should be replaced in the future.

smack-integration-test/src/main/java/org/igniterealtime/smack/inttest/AbstractSmackIntTest.java

@@ -61,14 +61,13 @@ public abstract class AbstractSmackIntTest {
         }
     }
 
-    @SuppressWarnings("ThreadPriorityCheck")
     protected void waitUntilTrue(Condition condition) throws TimeoutException, NoResponseException, XMPPErrorException, NotConnectedException, InterruptedException {
         final long deadline = System.currentTimeMillis() + timeout;
         do {
             if (condition.evaluate()) {
                 return;
             }
-            Thread.yield();
+            Thread.sleep(15);
         } while (System.currentTimeMillis() <= deadline);
         throw new TimeoutException("Timeout waiting for condition to become true. Timeout was " + timeout + " ms.");
     }