[lworld] RFR: 8372038: [lworld] Update ExplodedImage to use preview mode correctly [v3]

[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.

David Beaumont has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains eight additional commits since the last revision:

 - Merge branch 'lworld' into jdk_8372038_exploded/squashed
 - fixing duplicate elements
 - 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:
  - all: https://git.openjdk.org/valhalla/pull/1743/files
  - new: https://git.openjdk.org/valhalla/pull/1743/files/9abfa314..a01a4860

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1743&range=02
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1743&range=01-02

  Stats: 62700 lines in 778 files changed: 34276 ins; 23735 del; 4689 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