RFR: 8253935: [testbug] ComboBoxTest.testEditorKeyInputsWhenPopupIsShowing fails on Mac, Linux

[Ambarish Rapte]

The test fails on Mac and Linux.
Cause of failure:
- Linux: Unlike Windows, on Linux CTRL + Arrow keys move the caret to end/beginning of current word and not to
  beginning/end of next word.
- Mac: On Mac correct keys to traverse through words is ALT + Arrow and not CTRL + Arrow. And like on Linux, ALT + Arrow
  keys move the caret to end/beginning of current word and not to beginning/end of next word.

Fix is to use correct keys and asserts according to platform.

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

Commit messages:
 - test fix

Changes: https://git.openjdk.java.net/jfx/pull/317/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=317&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253935
  Stats: 42 lines in 1 file changed: 20 ins; 0 del; 22 mod
  Patch: https://git.openjdk.java.net/jfx/pull/317.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/317/head:pull/317

PR: https://git.openjdk.java.net/jfx/pull/317