During code review of [JDK-8346239](https://bugs.openjdk.org/browse/JDK-8346239) a few comments were made after the PR integrated. This follow-up patch cleans this up and adds a unit test for the `JimageDiffGenerator` class.
Testing: - [ ] GHA - [x] tools/jlink tests (including the new one). Thoughts? ------------- Commit messages: - 8347334: JimageDiffGenerator code clean-ups Changes: https://git.openjdk.org/jdk/pull/23014/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23014&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8347334 Stats: 319 lines in 2 files changed: 291 ins; 12 del; 16 mod Patch: https://git.openjdk.org/jdk/pull/23014.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/23014/head:pull/23014 PR: https://git.openjdk.org/jdk/pull/23014
