RFR: 8367915: [TestBug] Add system property to ScreenCaptureTestWatcher [v2]

[Andy Goryachev]

> Added `test.screenshot` system property to the `ScreenCaptureTestWatcher` to control writing of the screenshot data url to stderr.
> 
> Added corresponding gradle option `TEST_SCREENSHOT`.
> 
> The function is disabled by default for privacy reasons.

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

  firefox

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1910/files
  - new: https://git.openjdk.org/jfx/pull/1910/files/1284a09a..06603eab

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1910&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1910&range=00-01

  Stats: 5 lines in 1 file changed: 3 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/1910.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1910/head:pull/1910

PR: https://git.openjdk.org/jfx/pull/1910