RFR: 8206430: Use consistent pattern for startup in FX system tests [v3]

[Andy Goryachev]

> 1. Introduced the following utility methods:
> - Util.launch
> - Util.shutdown
> - Util.waitForLatch
> 2. Fixed the out-of order calls to Stage.hide() and Platform.exit() in many tests' shutdowns.
> 3. Replaced local waitForLatch copies with Util.waitForLatch

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

  8206430: default timeout 15

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Changes:
  - all: https://git.openjdk.org/jfx/pull/950/files
  - new: https://git.openjdk.org/jfx/pull/950/files/dbddbfad..57228341

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=950&range=02
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=950&range=01-02

  Stats: 109 lines in 86 files changed: 19 ins; 2 del; 88 mod
  Patch: https://git.openjdk.org/jfx/pull/950.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/950/head:pull/950

PR: https://git.openjdk.org/jfx/pull/950