RFR: 8325566: [TestBug] Util.shutdown() to hide ALL the scenes

[Andy Goryachev]

Changing `Util.shutdown()` to hide **all** showing Windows and then call `Platform.exit()`.
This simplifies the tests and ensures a clean shutdown.

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Commit messages:
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 - 8325566: [TestBug] Util.shutdown() to hide ALL the scenes

Changes: https://git.openjdk.org/jfx/pull/1407/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1407&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325566
  Stats: 124 lines in 59 files changed: 1 ins; 6 del; 117 mod
  Patch: https://git.openjdk.org/jfx/pull/1407.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1407/head:pull/1407

PR: https://git.openjdk.org/jfx/pull/1407