RFR: 8325566: [TestBug] Util.shutdown() to hide ALL Windows [v2]

[Andy Goryachev]

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

Andy Goryachev has updated the pull request incrementally with one additional commit since the last revision:

  for each

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1407/files
  - new: https://git.openjdk.org/jfx/pull/1407/files/2887d654..5773ebfd

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1407&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1407&range=00-01

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 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