[lworld] RFR: 8372038: [lworld] Update ExplodedImage to use preview mode correctly

[David Beaumont]

Adds preview mode support for ExplodedImage.

Adds tests so both ExplodedImage and ImageReader implementations of SystemImage are tested with exactly the same test data and assertions.

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Commit messages:
 - end-of-file newline
 - comment fix
 - Tweaking tests to explain jtreg issue
 - delete old tests
 - tests unified in SystemImage, all passing
 - Exploded image parity of testing

Changes: https://git.openjdk.org/valhalla/pull/1743/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1743&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372038
  Stats: 1082 lines in 7 files changed: 705 ins; 309 del; 68 mod
  Patch: https://git.openjdk.org/valhalla/pull/1743.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1743/head:pull/1743

PR: https://git.openjdk.org/valhalla/pull/1743