<Swing Dev> RFR: 8258924: javax/swing/JSplitPane/4201995/bug4201995.java fails in GTk L&F

[Prasanta Sadhukhan]

This test tests whether SplitPane is opaque or not but in nimbus and subseqeuently in GTK L&F the opaque property is set to false [1]
As nimbus is already handled in the test, it is modified to handle GTK L&F too. Along with this modification, the test is also updated to test all installed L&Fs.
Mach5 job running for several iterations for all platform is ok. Link in JBS.

[1] https://raw.githubusercontent.com/openjdk/jdk/master/src/java.desktop/share/classes/javax/swing/plaf/nimbus/skin.laf
      uiComponent opaque="false" type="javax.swing.JSplitPane" name="SplitPane" ui="SplitPaneUI" subregion="false"

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Commit messages:
 - gtk fix

Changes: https://git.openjdk.java.net/jdk/pull/1888/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1888&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258924
  Stats: 25 lines in 2 files changed: 15 ins; 1 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1888.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1888/head:pull/1888

PR: https://git.openjdk.java.net/jdk/pull/1888