RFR: 8338041: Keyborad Navigation of JTable, Ctrl Shift RIGHT/LEFT doesn't follow native action in GTK L&F

[Tejesh R]

In JTable, keyboard navigation keys Ctrl Shift RIGHT/LEFT doesn't follow native actions of Linux. In native the actions are extended to end of selected columns cells either LEFT/RIGHT but in swing gtk look and feel the selection is extended to one cell to left/right. This might be taken as reference of Windows OS since the same is observed in Windows native. Hence I have update the actions for Ctrl Shift RIGHT & LEFT. 
Added automated test too. The fix is tested in CI and its fine.

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Commit messages:
 - Test updates
 - Key action modified + Automatic test added

Changes: https://git.openjdk.org/jdk/pull/20608/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20608&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338041
  Stats: 140 lines in 2 files changed: 138 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/20608.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20608/head:pull/20608

PR: https://git.openjdk.org/jdk/pull/20608