[FLINK-8262] [tests] Harden IndividualRestartsConcurrencyTest.testLoc…

…alFailureFailsPendingCheckpoints

The problem was a concurrent restart attempt which failed due to not enough
available slots. This failure would lead to the job failure and the discarding
of all pending checkpoints.

flink-runtime/src/test/java/org/apache/flink/runtime/executiongraph/IndividualRestartsConcurrencyTest.java

@@ -305,8 +305,7 @@ public void testLocalFailureFailsPendingCheckpoints() throws Exception {
  when(taskManagerGateway.cancelTask(any(ExecutionAttemptID.class), any(Time.class))).thenReturn(CompletableFuture.completedFuture(Acknowledge.get()));
 
  final SimpleSlotProvider slotProvider = new SimpleSlotProvider(jid, parallelism, taskManagerGateway);
- final Executor executor = TestingUtils.defaultExecutor();
-
+ final ManuallyTriggeredDirectExecutor executor = new ManuallyTriggeredDirectExecutor();
 
  final CheckpointCoordinatorConfiguration checkpointCoordinatorConfiguration = new CheckpointCoordinatorConfiguration(
  10L,