RFR: 8206430: Use consistent pattern for startup in FX system tests [v6]

[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: review comments

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Changes:
  - all: https://git.openjdk.org/jfx/pull/950/files
  - new: https://git.openjdk.org/jfx/pull/950/files/3fe70c20..4fd492e5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=950&range=05
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=950&range=04-05

  Stats: 13 lines in 1 file changed: 4 ins; 0 del; 9 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