<Swing Dev> RFR: 8252194: Add automated test for fix done in JDK-8218469

[Pankaj Bansal]

Under JDK-8218469, fix was made to correct the rendering of JSlider as the Slider knob/head was not being rendered at
all. The reason was that gtk3 changed the way styles are used. No automated test was written to verify the results.
Current bug is to write the automated tests for the same. I have verified that the test fails without the fix done for
DK-8218469 and passes with the fix.

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Commit messages:
 - Add automated test for fix done in JDK-8218469

Changes: https://git.openjdk.java.net/jdk/pull/374/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=374&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252194
  Stats: 148 lines in 1 file changed: 148 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/374.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/374/head:pull/374

PR: https://git.openjdk.java.net/jdk/pull/374