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

[Ajit Ghaisas]

On Wed, 16 Nov 2022 20:12:26 GMT, Andy Goryachev <angorya at openjdk.org> wrote:

>> 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

Marked as reviewed by aghaisas (Reviewer).

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