RFR: 8327478: Add System test to verify TextSelection issue for webkit-617.1

[Gopal Pattnaik]

There was no test included with the fix for [JDK-8326989](https://bugs.openjdk.org/browse/JDK-8326989),

Hence we are adding a system test now.

Test is written as 
1. Load html content in web view.
2. pick the color of mouse pointer.
3. Perform double click.
4. pick the color again.
> expected bahaviour: colour picked in step 2 and 4 should not match.

Verification:
The test passes with latest webkit source
Also verified that test fails when the fix for [JDK-8326989](https://bugs.openjdk.org/browse/JDK-8326989) is reverted.

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Commit messages:
 - Addressed Review comments
 - Empty Commit to test actions.
 - Addressed Review comments
 - Add System Test TextSelectionTest

Changes: https://git.openjdk.org/jfx/pull/1719/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1719&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327478
  Stats: 126 lines in 2 files changed: 126 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1719.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1719/head:pull/1719

PR: https://git.openjdk.org/jfx/pull/1719