[jdk17u] RFR: 8269269: [macos11] SystemIconTest fails with ClassCastException

[Alex Kasko]

Clean backport of the test-only Mac-specific fix.

Testing: checked that the test fails on Mac without the patch, checked that changed test passes on Linux, Windows and Mac.

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Commit messages:
 - Backport eab959cbfa9e781adac3bc4bc3e74af8b18a1fd5

Changes: https://git.openjdk.java.net/jdk17u/pull/142/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17u&pr=142&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269269
  Stats: 21 lines in 1 file changed: 13 ins; 4 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk17u/pull/142.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17u pull/142/head:pull/142

PR: https://git.openjdk.java.net/jdk17u/pull/142