[SPARK-53712] Fix spark-operator to use JUnit 5 instead of JUnit 4

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

This PR aims to fix `spark-operator` module to use `JUnit 5` correctly instead of the transitive `JUnit 4` dependency.

### Why are the changes needed?

`Apache Spark K8s Operator` should use `JUnit 5` consistently.

https://github.com/apache/spark-kubernetes-operator/blob/a0929a4023f7a0f9eba5765fdb47e17228d68a0e/gradle/libs.versions.toml#L26

**BEFORE**

```
$ gradle :spark-operator:dependencies | grep ' junit:junit' | wc -l
       3
```

**AFTER**

```
$ gradle :spark-operator:dependencies | grep ' junit:junit' | wc -l
       0
```

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

No, this is a test code and dependency change.

### How was this patch tested?

Pass the CIs.

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

No.

Closes apache#344 from dongjoon-hyun/SPARK-53712.

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

Author: dongjoon-hyun

spark-operator/build.gradle

@@ -65,7 +65,9 @@ dependencies {
     exclude group: "org.apache.logging.log4j"
     exclude group: "org.slf4j"
   }
-  testImplementation(libs.mockwebserver)
+  testImplementation(libs.mockwebserver) {
+    exclude group: 'junit'
+  }
   testImplementation platform(libs.junit.bom)
   testImplementation(libs.junit.jupiter)
   testRuntimeOnly(libs.junit.platform.launcher)

spark-operator/src/test/java/org/apache/spark/k8s/operator/metrics/MetricsSystemFactoryTest.java

@@ -20,7 +20,7 @@
 package org.apache.spark.k8s.operator.metrics;
 
 import static org.apache.spark.k8s.operator.metrics.MetricsSystemFactory.parseSinkProperties;
-import static org.junit.Assert.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import java.util.Properties;
 

spark-operator/src/test/java/org/apache/spark/k8s/operator/metrics/source/KubernetesMetricsInterceptorTest.java

@@ -19,7 +19,7 @@
 
 package org.apache.spark.k8s.operator.metrics.source;
 
-import static org.junit.Assert.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import java.util.Arrays;
 import java.util.Collections;