RFR: 8206430: Use consistent pattern for startup in FX system tests [v2]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 11 additional commits since the last revision:

 - 8206430: run and wait
 - Merge remote-tracking branch 'origin/master' into 8206430.init.fx
 - 8206430: wait for latch
 - 8206430: exit test
 - 8206430: wait for latch
 - 8206430: t..w
 - 8206430: r..s
 - 8206430: h..p
 - 8206430: util.shutdown
 - 8206430: c..f
 - ... and 1 more: https://git.openjdk.org/jfx/compare/4f924f96...dbddbfad

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/950/files
  - new: https://git.openjdk.org/jfx/pull/950/files/51023c47..dbddbfad

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=950&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=950&range=00-01

  Stats: 8 lines in 2 files changed: 0 ins; 7 del; 1 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