[SPARK-26482][K8S][TEST][FOLLOWUP] Fix compile failure

## What changes were proposed in this pull request?

This fixes K8S integration test compilation failure introduced by #23423 .
```scala
$ build/sbt -Pkubernetes-integration-tests test:package
...
[error] /Users/dongjoon/APACHE/spark/resource-managers/kubernetes/integration-tests/src/test/scala/org/apache/spark/deploy/k8s/integrationtest/KubernetesTestComponents.scala:71: type mismatch;
[error]  found   : org.apache.spark.internal.config.OptionalConfigEntry[Boolean]
[error]  required: String
[error]       .set(IS_TESTING, false)
[error]            ^
[error] /Users/dongjoon/APACHE/spark/resource-managers/kubernetes/integration-tests/src/test/scala/org/apache/spark/deploy/k8s/integrationtest/KubernetesTestComponents.scala:71: type mismatch;
[error]  found   : Boolean(false)
[error]  required: String
[error]       .set(IS_TESTING, false)
[error]                        ^
[error] two errors found
```

## How was this patch tested?

Pass the K8S integration test.

Closes #23527 from dongjoon-hyun/SPARK-26482.

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/KubernetesTestComponents.scala

@@ -68,8 +68,8 @@ private[spark] class KubernetesTestComponents(defaultClient: DefaultKubernetesCl
       .set("spark.executor.cores", "1")
       .set("spark.executors.instances", "1")
       .set("spark.app.name", "spark-test-app")
-      .set(IS_TESTING, false)
-      .set(UI_ENABLED, true)
+      .set(IS_TESTING.key, "false")
+      .set(UI_ENABLED.key, "true")
       .set("spark.kubernetes.submission.waitAppCompletion", "false")
       .set("spark.kubernetes.authenticate.driver.serviceAccountName", serviceAccountName)
   }