[SPARK-49381][K8S][TESTS] Fix SPARK-41388 test case to assert the result

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

This PR aims to fix a test case of SPARK-41388 by adding a missed assert.

### Why are the changes needed?

To check the result correctly.

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

No, this is a test case change.

### How was this patch tested?

Pass the CIs with newly added assertion.

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

No.

Closes #47865 from dongjoon-hyun/SPARK-49381.

Authored-by: Dongjoon Hyun <dongjoon@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

Author: dongjoon-hyun

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

@@ -845,15 +845,17 @@ class ExecutorPodsAllocatorSuite extends SparkFunSuite with BeforeAndAfter {
     val getReusablePVCs =
       PrivateMethod[mutable.Buffer[PersistentVolumeClaim]](Symbol("getReusablePVCs"))
 
-    val pvc1 = persistentVolumeClaim("pvc-0", "gp2", "200Gi")
-    val pvc2 = persistentVolumeClaim("pvc-1", "gp2", "200Gi")
+    val pvc1 = persistentVolumeClaim("pvc-1", "gp2", "200Gi")
+    val pvc2 = persistentVolumeClaim("pvc-2", "gp2", "200Gi")
 
     val now = Instant.now()
     pvc1.getMetadata.setCreationTimestamp(now.minus(2 * podAllocationDelay, MILLIS).toString)
     pvc2.getMetadata.setCreationTimestamp(now.toString)
 
     when(persistentVolumeClaimList.getItems).thenReturn(Seq(pvc1, pvc2).asJava)
-    podsAllocatorUnderTest invokePrivate getReusablePVCs("appId", Seq("pvc-1"))
+    val reusablePVCs = podsAllocatorUnderTest invokePrivate getReusablePVCs("appId", Seq.empty)
+    assert(reusablePVCs.size == 1)
+    assert(reusablePVCs.head.getMetadata.getName == "pvc-1")
   }
 
   test("SPARK-41410: Support waitToReusePersistentVolumeClaims") {