RFR: 8206430: Use consistent pattern for startup in FX system tests [v4]

[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: util startup

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/950/files
  - new: https://git.openjdk.org/jfx/pull/950/files/57228341..7560c0f0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=950&range=03
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=950&range=02-03

  Stats: 233 lines in 11 files changed: 82 ins; 88 del; 63 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