RFR: 8294046: Newly added test test/jdk/javax/swing/JTabbedPane/TestNPEStateChgListener.java fails in macos

[Prasanta Sadhukhan]

Test added for fix [JDK-6286501](https://bugs.openjdk.org/browse/JDK-6286501)  fails in macos citing 

java.lang.NullPointerException: Cannot invoke "javax.swing.JTabbedPane.getSelectedIndex()" 
because "this.this$0.tabPane" is null
at java.desktop/com.apple.laf.AquaTabbedPaneUI$2.stateChanged(AquaTabbedPaneUI.java:813) 


for Aqua L&F owing to same issue as described in https://github.com/openjdk/jdk/pull/10216#discussion_r971787291 wherein the old instance is uninstalled and tabPane is reset to null when updateUI() is called in stateChange Listener in testcase, so we need to use the tabPane instance passed in TabbedPane's stateChange listener as has been done for BasicTabbedPaneUI case.

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Commit messages:
 - 8294046: Newly added test test/jdk/javax/swing/JTabbedPane/TestNPEStateChgListener.java fails in macos

Changes: https://git.openjdk.org/jdk/pull/10354/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10354&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294046
  Stats: 2 lines in 2 files changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/10354.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10354/head:pull/10354

PR: https://git.openjdk.org/jdk/pull/10354