[SPARK-53324][K8S][FOLLOWUP] Use invokePrivate to access numOutstandingPods

### What changes were proposed in this pull request?

This is a follow-up of the following to apply the latest `master` branch change.
- apache#51913

### Why are the changes needed?

The master branch compilation is broken currently.
- https://github.com/apache/spark/actions/runs/18760939731

Recently, we changed `numOutstandingPods`'s visibility, but I missed that the old open PRs didn't have this change.
- apache#52614

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Pass the CIs.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes apache#52714 from dongjoon-hyun/SPARK-53324.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

resource-managers/kubernetes/core/src/test/scala/org/apache/spark/scheduler/cluster/k8s/ExecutorPodsAllocatorSuite.scala

@@ -273,7 +273,7 @@ class ExecutorPodsAllocatorSuite extends SparkFunSuite with BeforeAndAfter {
     // Request more than the max per rp for one rp
     podsAllocatorUnderTest.setTotalExpectedExecutors(Map(defaultProfile -> 2, rp -> 3))
     // 2 for default, and 2 for rp
-    assert(podsAllocatorUnderTest.numOutstandingPods.get() == 4)
+    assert(podsAllocatorUnderTest.invokePrivate(numOutstandingPods).get() == 4)
     verify(podsWithNamespace).resource(podWithAttachedContainerForId(1, defaultProfile.id))
     verify(podsWithNamespace).resource(podWithAttachedContainerForId(2, defaultProfile.id))
     verify(podsWithNamespace).resource(podWithAttachedContainerForId(3, rp.id))
@@ -285,7 +285,7 @@ class ExecutorPodsAllocatorSuite extends SparkFunSuite with BeforeAndAfter {
     snapshotsStore.updatePod(pendingExecutor(2, defaultProfile.id))
     snapshotsStore.updatePod(pendingExecutor(3, rp.id))
     snapshotsStore.notifySubscribers()
-    assert(podsAllocatorUnderTest.numOutstandingPods.get() == 4)
+    assert(podsAllocatorUnderTest.invokePrivate(numOutstandingPods).get() == 4)
     verify(podResource, times(4)).create()
     verify(labeledPods, never()).delete()
 
@@ -294,14 +294,14 @@ class ExecutorPodsAllocatorSuite extends SparkFunSuite with BeforeAndAfter {
     waitForExecutorPodsClock.advance(executorIdleTimeout * 2)
     podsAllocatorUnderTest.setTotalExpectedExecutors(Map(defaultProfile -> 1, rp -> 3))
     snapshotsStore.notifySubscribers()
-    assert(podsAllocatorUnderTest.numOutstandingPods.get() == 3)
+    assert(podsAllocatorUnderTest.invokePrivate(numOutstandingPods).get() == 3)
     verify(labeledPods, times(1)).delete()
 
     // Make one pod running from non-default rp so we have one more slot for pending pods.
     snapshotsStore.updatePod(runningExecutor(3, rp.id))
     snapshotsStore.updatePod(pendingExecutor(4, rp.id))
     snapshotsStore.notifySubscribers()
-    assert(podsAllocatorUnderTest.numOutstandingPods.get() == 3)
+    assert(podsAllocatorUnderTest.invokePrivate(numOutstandingPods).get() == 3)
     verify(podsWithNamespace).resource(podWithAttachedContainerForId(5, rp.id))
     verify(labeledPods, times(1)).delete()
   }