RFR: 8365436: ImageReaderTest fails when jmods directory not present

[David Beaumont]

Changed test helper creation to know about linkable images, and changed how test skipping is reported (JUnit doesn't honor TestSkippedException, only TestAbortedException).

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Commit messages:
 - Make test helper aware of linkable images, and fix how test skipping occurs.

Changes: https://git.openjdk.org/jdk/pull/26774/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26774&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365436
  Stats: 12 lines in 1 file changed: 6 ins; 4 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/26774.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26774/head:pull/26774

PR: https://git.openjdk.org/jdk/pull/26774