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

[dongjoon-hyun]

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.

spark-kubernetes-operator/gradle/libs.versions.toml

Line 26 in a0929a4

junit = "5.13.4"

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.

[dongjoon-hyun]

Thank you, @peter-toth .

[dongjoon-hyun]

Merged to main.