[SPARK-55173][K8S][TESTS] Improve K8s IT to use TestConstants consistently and match syntax for extensibility

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

This PR aims to improve K8s integration tests by refactoring
- To re-use `TestConstants` consistently, e.g., `"minikube"` -> `BACKEND_MINIKUBE` and `"docker-desktop"` -> `BACKEND_DOCKER_DESKTOP`.
- To use `match` syntax (instead of `if` statement) for extensibility.

```scala
-    val storageClassName = if (testBackend == MinikubeTestBackend) "standard" else "hostpath"
-    val hostname = if (testBackend == MinikubeTestBackend) "minikube" else "docker-desktop"
+    val (storageClassName, hostname) = testBackend match {
+      case MinikubeTestBackend => ("standard", BACKEND_MINIKUBE)
+      case DockerForDesktopBackend => ("hostpath", BACKEND_DOCKER_DESKTOP)
+      case _ => ("hostpath", BACKEND_DOCKER_DESKTOP)
+    }
```

### Why are the changes needed?

This helps us maintain the test backend consistently and add a new backend easily.

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

No, this is a test-only change.

### How was this patch tested?

Pass the CIs with the existing test cases.

Manually check the previous code patterns like the following.

**BEFORE**

```
$ git grep 'testBackend ==' | wc -l
       4
```

```
$ git grep "docker-desktop" | grep scala
resource-managers/kubernetes/integration-tests/src/test/scala/org/apache/spark/deploy/k8s/integrationtest/PVTestsSuite.scala:    val hostname = if (testBackend == MinikubeTestBackend) "minikube" else "docker-desktop"
resource-managers/kubernetes/integration-tests/src/test/scala/org/apache/spark/deploy/k8s/integrationtest/TestConstants.scala:  val BACKEND_DOCKER_DESKTOP = "docker-desktop"
```

**AFTER**

```
$ git grep 'testBackend ==' | wc -l
       0
```

```
$ git grep "docker-desktop" | grep scala
resource-managers/kubernetes/integration-tests/src/test/scala/org/apache/spark/deploy/k8s/integrationtest/TestConstants.scala:  val BACKEND_DOCKER_DESKTOP = "docker-desktop"
```

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

No.

Closes #53956 from dongjoon-hyun/SPARK-55173.

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

Author: dongjoon-hyun

resource-managers/kubernetes/integration-tests/src/test/scala/org/apache/spark/deploy/k8s/integrationtest/PVTestsSuite.scala

@@ -23,14 +23,19 @@ import org.scalatest.concurrent.{Eventually, PatienceConfiguration}
 import org.scalatest.time.{Milliseconds, Span}
 
 import org.apache.spark.deploy.k8s.integrationtest.KubernetesSuite._
+import org.apache.spark.deploy.k8s.integrationtest.TestConstants._
+import org.apache.spark.deploy.k8s.integrationtest.backend.docker.DockerForDesktopBackend
 import org.apache.spark.deploy.k8s.integrationtest.backend.minikube.MinikubeTestBackend
 
 private[spark] trait PVTestsSuite { k8sSuite: KubernetesSuite =>
   import PVTestsSuite._
 
   private def setupLocalStorage(): Unit = {
-    val storageClassName = if (testBackend == MinikubeTestBackend) "standard" else "hostpath"
-    val hostname = if (testBackend == MinikubeTestBackend) "minikube" else "docker-desktop"
+    val (storageClassName, hostname) = testBackend match {
+      case MinikubeTestBackend => ("standard", BACKEND_MINIKUBE)
+      case DockerForDesktopBackend => ("hostpath", BACKEND_DOCKER_DESKTOP)
+      case _ => ("hostpath", BACKEND_DOCKER_DESKTOP)
+    }
     val pvBuilder = new PersistentVolumeBuilder()
       .withKind("PersistentVolume")
       .withApiVersion("v1")

resource-managers/kubernetes/integration-tests/src/test/scala/org/apache/spark/deploy/k8s/integrationtest/VolumeSuite.scala

@@ -99,7 +99,10 @@ private[spark] trait VolumeSuite { k8sSuite: KubernetesSuite =>
   }
 
   test("A driver-only Spark job with an OnDemand PVC volume", k8sTestTag) {
-    val storageClassName = if (testBackend == MinikubeTestBackend) "standard" else "hostpath"
+    val storageClassName = testBackend match {
+      case MinikubeTestBackend => "standard"
+      case _ => "hostpath"
+    }
     val DRIVER_PREFIX = "spark.kubernetes.driver.volumes.persistentVolumeClaim"
     sparkAppConf
       .set("spark.kubernetes.driver.master", "local[10]")
@@ -148,7 +151,10 @@ private[spark] trait VolumeSuite { k8sSuite: KubernetesSuite =>
   }
 
   test("A Spark job with two executors with OnDemand PVC volumes", k8sTestTag) {
-    val storageClassName = if (testBackend == MinikubeTestBackend) "standard" else "hostpath"
+    val storageClassName = testBackend match {
+      case MinikubeTestBackend => "standard"
+      case _ => "hostpath"
+    }
     val EXECUTOR_PREFIX = "spark.kubernetes.executor.volumes.persistentVolumeClaim"
     sparkAppConf
       .set("spark.executor.instances", "2")

resource-managers/kubernetes/integration-tests/src/test/scala/org/apache/spark/deploy/k8s/integrationtest/backend/minikube/Minikube.scala

@@ -19,6 +19,7 @@ package org.apache.spark.deploy.k8s.integrationtest.backend.minikube
 import io.fabric8.kubernetes.client.{Config, KubernetesClient, KubernetesClientBuilder}
 
 import org.apache.spark.deploy.k8s.integrationtest.ProcessUtils
+import org.apache.spark.deploy.k8s.integrationtest.TestConstants._
 import org.apache.spark.internal.Logging
 
 // TODO support windows
@@ -57,7 +58,7 @@ private[spark] object Minikube extends Logging {
           "non-numeric suffix is intentionally dropped)")
     }
 
-    new KubernetesClientBuilder().withConfig(Config.autoConfigure("minikube")).build()
+    new KubernetesClientBuilder().withConfig(Config.autoConfigure(BACKEND_MINIKUBE)).build()
   }
 
   def getMinikubeStatus(): MinikubeStatus.Value = {