RFR: 8359598: [TestBug] VirtualFlowTestUtils should not create a temporary Stage [v2]

[Marius Hanl]

> Currently, the VirtualFlowTestUtils used ONLY for tests has many utility methods to get and do something with the VirtualFlow of Virtualized Controls.
> 
> It has one flaw: It may creates a temporary Stage with the StageLoader, when the Control was never inside a Scene (and Stage) yet. This is done to get the VirtualFlow, which is not possible otherwise.
> 
> Take the following test code:
> 
> VirtualFlowTestUtils.clickOnRow(tableView, 2);
> VirtualFlowTestUtils.clickOnRow(tableView, 4);
> 
> 
> What it does it to create a Stage for the first method and dispose it after. And create another Stage for the second method and dispose it after.
> 
> This does not test a realistic scenario and the chance is quite high that developers used that methods without even knowing that it contains such logic.
> 
> So the idea is to remove the StageLoader code from VirtualFlowTestUtils and rather use it in the Test code before calling the Util methods.
> 
> For the example above, this would result in:
> 
> stageLoader = new StageLoader(tableView);
> VirtualFlowTestUtils.clickOnRow(tableView, 2);
> VirtualFlowTestUtils.clickOnRow(tableView, 4);
> 
> 
> The stageLoader should be disposed in an @AfterEach.
> 
> Note: Nearly all touched tests are indeed much older test code. New tests are not affected and already use it correcty.
> Sometimes a call to `Toolkit.getToolkit().firePulse();` was needed. Previously, creating a new Stage for every call to the util methods did that implicitly.

Marius Hanl has updated the pull request incrementally with one additional commit since the last revision:

  Exception message

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1829/files
  - new: https://git.openjdk.org/jfx/pull/1829/files/5cde6fd6..8c5d40eb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1829&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1829&range=00-01

  Stats: 5 lines in 1 file changed: 5 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1829.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1829/head:pull/1829

PR: https://git.openjdk.org/jfx/pull/1829