RFR: 8289606: CustomSecurityManagerTest fails on Mac M1

[Andy Goryachev]

- sampling at 1/3 of screen height instead of the very top to avoid hitting OS UI found on the newer Macs with a notch
- change applies to all platforms

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Commit messages:
 - 8289606: fixed CustomSecurityManagerTest fails on Mac M1

Changes: https://git.openjdk.org/jfx/pull/826/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=826&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289606
  Stats: 10 lines in 1 file changed: 8 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/826.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/826/head:pull/826

PR: https://git.openjdk.org/jfx/pull/826