Fix banned imports in TestParquetFileReaderBufferLeak (since GH-3487)

[abstractdog]

Rationale for this change

Eariler commit makes precommit fail as: https://github.com/apache/parquet-java/actions/runs/33898249347/job/101105940903?pr=3773

[ERROR] Reason: Use JUnit 5 (org.junit.jupiter.*) and AssertJ instead of JUnit 4
[ERROR] 	in file:///home/runner/work/parquet-java/parquet-java/parquet-hadoop/src/test/java/org/apache/parquet/hadoop/TestParquetFileReaderBufferLeak.java
[ERROR] 		static org.junit.Assert.assertEquals  	(Line: 23, Matched by: org.junit.**)
[ERROR] 		static org.junit.Assert.assertNotNull 	(Line: 24, Matched by: org.junit.**)
[ERROR] 		static org.junit.Assert.assertNull    	(Line: 25, Matched by: org.junit.**)
[ERROR] 		static org.junit.Assert.assertTrue    	(Line: 26, Matched by: org.junit.**)
[ERROR] 		org.junit.Rule                        	(Line: 47, Matched by: org.junit.**)
[ERROR] 		org.junit.Test                        	(Line: 48, Matched by: org.junit.**)
[ERROR] 		org.junit.rules.TemporaryFolder       	(Line: 49, Matched by: org.junit.**)

What changes are included in this PR?

Fixing the banned imports.

Are these changes tested?

Yes, tests passed.

Are there any user-facing changes?

No.

[abstractdog]

cc @arouel , @Fokko: as far as I can see this got broken today

[Fokko]

@abstractdog I forgot about the banned imports 😨 Thanks for fixing this, appreciate it.

[abstractdog]

@abstractdog I forgot about the banned imports 😨 Thanks for fixing this, appreciate it.

sure, happy to help here!