RFR: 8367991: Update RegionBackground tests to use ScreenCaptureTestWatcher utility

[Jayathirth D V]

We are seeing very rare intermittent failures in RegionBackgroundFillUITest & RegionBackgroundImageUITest tests in linux. We already have [JDK-8328217](https://bugs.openjdk.org/browse/JDK-8328217) to fix this issue.

To identify the root cause better, we want to capture the test content when it is failing using ScreenCaptureTestWatcher utility.

Since we wan to capture the test window before test stage is cleared. We are now clearing the test stage and next test setup. Initially tried updating this behaviour in `VisualTestBase` itself, but that causes tests like `test.robot.test3d.PointLightIlluminationTest` as it overrides the test setup functions.

This change is tested to make sure ScreenCaptureTestWatcher takes appropriate screen capture on failure and CI full headful test run is green.

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Commit messages:
 - refactor
 - 8367991: Update RegionBackground tests to use ScreenCaptureTestWatcher utility

Changes: https://git.openjdk.org/jfx/pull/1913/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1913&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367991
  Stats: 96 lines in 4 files changed: 86 ins; 5 del; 5 mod
  Patch: https://git.openjdk.org/jfx/pull/1913.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1913/head:pull/1913

PR: https://git.openjdk.org/jfx/pull/1913