<Swing Dev> [jdk17] RFR: 8269269: [macos11] SystemIconTest fails with ClassCastException

[Alexander Zuev]

Added check for the icon class type
Added check if file or folder we are testing against exists and
accessible
Removed test from problem list

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Commit messages:
 - 8269269: [macos11] SystemIconTest fails with ClassCastException

Changes: https://git.openjdk.java.net/jdk17/pull/178/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17&pr=178&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269269
  Stats: 16 lines in 2 files changed: 12 ins; 1 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk17/pull/178.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17 pull/178/head:pull/178

PR: https://git.openjdk.java.net/jdk17/pull/178