RFR: 8196465: javax/swing/JComboBox/8182031/ComboPopupTest.java fails on Linux

[Prasanta Sadhukhan]

Test was seen to be failing in linux CI system although passing locally.
Added waitForIdle(). 
Test is now passing in all CI systems including linux and mac. Several iterations of the test in CI run is ok (link in JBS)

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Commit messages:
 - 8196465: javax/swing/JComboBox/8182031/ComboPopupTest.java fails on Linux

Changes: https://git.openjdk.java.net/jdk/pull/8197/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8197&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8196465
  Stats: 29 lines in 2 files changed: 6 ins; 18 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8197.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8197/head:pull/8197

PR: https://git.openjdk.java.net/jdk/pull/8197