<Swing Dev> RFR: 8196465: javax/swing/JComboBox/8182031/ComboPopupTest.java fails on Linux

[Prasanta Sadhukhan]

Please review a test fix for an issue seen to be failing in mach5 testing on linux whereby the combo popup is not opening.
The issue is not reproducible locally in ubuntu18.04 but mach5 testing on ubuntu18.04 was able to see the issue. 
Combo popup was opened by clicking at right edge of combo box, which sometimes is not set properly at mach5 system, so mouse click is clicked at wrong location of frame, thereby not opening the combo popup.
 Modified the test to open the combo popup by clicking on middle of combobox which now works for all. Also, made robot autodelay consistent with other test to be 100ms. Also, changed deprecated constant to appropriate constants.
Mach5 job was run for several iteration on all 3 platforms. 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/829/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=829&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8196465
  Stats: 6 lines in 2 files changed: 1 ins; 1 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/829.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/829/head:pull/829

PR: https://git.openjdk.java.net/jdk/pull/829