RFR: 8365436: ImageReaderTest fails when jmods directory not present

[David Beaumont]

Switched from TestSkippedException (not honoured by JUnit) to TestAbortedException (via Assumptions class) to avoid false negative test "failure". Note that now, the test is either shown as skipped, or passed, depending on which report is being looked at. Tweaked error message slightly since using an "exploded image" isn't the only cause.

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Commit messages:
 - Remove unused imports.
 - Change how test is skipped to avoid false negative reporting.

Changes: https://git.openjdk.org/jdk/pull/26773/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26773&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365436
  Stats: 14 lines in 1 file changed: 8 ins; 5 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/26773.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26773/head:pull/26773

PR: https://git.openjdk.org/jdk/pull/26773