RFR: 8255337: [TestBug] Controls unit tests - ButtonTest and ComboBoxTest - log ClassCastException [v2]

[Ajit Ghaisas]

> This is a test fix.
> 
> Root cause:
> - For ButtonTest - Incorrect use of MouseEventFirer - which puts Button on to the stage and shows it before firing mouse event
> - For ComboBoxTest - adding ComboBox to a Stage, not showing the Stage but trying to show the ComboBox
> 
> 
> Fix :
> - For ButtonTest - Moved MouseEventFirer usage from test class to the 2 tests that need it
> - For ComboBoxTest - Started using StageLoader (which adds comboBox to the stage and shows it) instead of separate Scene and Stage creation.
> 
> 
> I have attached the logs captured before and after this fix to the JBS.

Ajit Ghaisas has updated the pull request incrementally with one additional commit since the last revision:

  review_fixes

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/337/files
  - new: https://git.openjdk.java.net/jfx/pull/337/files/8a6fc1cf..4826d814

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=337&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=337&range=00-01

  Stats: 151 lines in 2 files changed: 10 ins; 94 del; 47 mod
  Patch: https://git.openjdk.java.net/jfx/pull/337.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/337/head:pull/337

PR: https://git.openjdk.java.net/jfx/pull/337