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

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;