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

[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.

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Commit messages:
 - test.screenshot system property

Changes: https://git.openjdk.org/jfx/pull/1910/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1910&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367915
  Stats: 18 lines in 2 files changed: 15 ins; 0 del; 3 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