RFR: 8347334: JimageDiffGenerator code clean-ups [v2]

[Severin Gehwolf]

> 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?

Severin Gehwolf has updated the pull request incrementally with one additional commit since the last revision:

  Review feedback

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Changes:
  - all: https://git.openjdk.org/jdk/pull/23014/files
  - new: https://git.openjdk.org/jdk/pull/23014/files/5b98866d..42baf977

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23014&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23014&range=00-01

  Stats: 48 lines in 2 files changed: 42 ins; 2 del; 4 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