[FLINK-8138] [tests] Fix TaskAsyncCallTest#testSetsUserCodeClassLoader

The problem was a race condition between the triggerCheckpoint and the wait
loop which had as termination condition the number of received trigger checkpoint
messages.

flink-runtime/src/test/java/org/apache/flink/runtime/taskmanager/TaskAsyncCallTest.java

@@ -56,6 +56,7 @@
 import org.apache.flink.runtime.state.TestTaskStateManager;
 import org.apache.flink.runtime.util.TestingTaskManagerRuntimeInfo;
 import org.apache.flink.util.SerializedValue;
+import org.apache.flink.util.TestLogger;
 
 import org.junit.Before;
 import org.junit.Test;
@@ -78,7 +79,7 @@
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.when;
 
-public class TaskAsyncCallTest {
+public class TaskAsyncCallTest extends TestLogger {
 
 	/** Number of expected checkpoints. */
 	private static int numCalls;
@@ -289,7 +290,7 @@ public void invoke() throws Exception {
 
 			// wait forever (until canceled)
 			synchronized (this) {
-				while (error == null && lastCheckpointId < numCalls) {
+				while (error == null) {
 					wait();
 				}
 			}