RFR: 8347334: JimageDiffGenerator code clean-ups [v3]

[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:
> - [x] 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:

  Further simplification

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/23014/files
  - new: https://git.openjdk.org/jdk/pull/23014/files/42baf977..c9cbb9a0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23014&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23014&range=01-02

  Stats: 13 lines in 1 file changed: 1 ins; 7 del; 5 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