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

[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.

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Commit messages:
 - 8255337-fix-test-log-exceptions

Changes: https://git.openjdk.java.net/jfx/pull/337/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=337&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255337
  Stats: 50 lines in 2 files changed: 21 ins; 18 del; 11 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