[SPARK-53872] Revisit JUnit assert usage in test cases

[dongjoon-hyun]

What changes were proposed in this pull request?

This PR aims to revisit JUnit assertion usage in test cases after upgrading to JUnit 6.

• [SPARK-53871] Upgrade JUnit to 6.0.0 #383

Why are the changes needed?

There are three cases:

1. Fix wrong usage of assertEquals: The first parameter is the expected value.

- assertEquals(previousGeneration2, 1L);
+ assertEquals(1L, previousGeneration2);

2. assertNull is better than assertEquals(null, *).

- assertEquals(null, spec1.runtimeVersions.jdkVersion);
- assertEquals(null, spec1.runtimeVersions.scalaVersion);
- assertEquals(null, spec1.runtimeVersions.sparkVersion);
+ assertNull(spec1.runtimeVersions.jdkVersion);
+ assertNull(spec1.runtimeVersions.scalaVersion);
+ assertNull(spec1.runtimeVersions.sparkVersion);

3. assertInstanceOf is more intuitive than assertTrue(... instanceof ...).

- assertTrue(constructorArgs.get(conf).get(0) instanceof SparkConf);
+ assertInstanceOf(SparkConf.class, constructorArgs.get(conf).get(0));

Does this PR introduce any user-facing change?

No. This is a test case fix and improvement.

How was this patch tested?

Pass the CIs.

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

No.

[dongjoon-hyun]

Thank you, @viirya ! Merged to main.