RFR: 8367991: Update RegionBackground tests to use ScreenCaptureTestWatcher utility [v4]

[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 want to capture the test window before test stage is cleared. We are now clearing the test stage in the next test setup. Initially tried updating this behaviour in `VisualTestBase` itself, but that causes tests like `test.robot.test3d.PointLightIlluminationTest` to fail 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.

Jayathirth D V has updated the pull request incrementally with one additional commit since the last revision:

  Add flag to disable stage clearing

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1913/files
  - new: https://git.openjdk.org/jfx/pull/1913/files/9c1993c2..0fd10912

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1913&range=03
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1913&range=02-03

  Stats: 15 lines in 2 files changed: 7 ins; 1 del; 7 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