<Swing Dev> RFR: 8196090: javax/swing/JComboBox/6559152/bug6559152.java fails

[Prasanta Sadhukhan]

Test seems to fail in mach5 nightly testing when ran in a group of all JComboBox tests.
Modified few tests that were run prior to running this test by moving the frame to centre of screen, adjusting the event delay time to more consistent value in sync with other test and also made this test's frame gain focus by clicking on the frame, as it seems the frame does not get focus sometime so that keyevent does not get registered causing it to fail.
Ran bunch of JComboBox test that were run prior to this tests in normal run, along with this test, in mach5 which is ok. Link in JBS.

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Commit messages:
 - 8196090: javax/swing/JComboBox/6559152/bug6559152.java fails

Changes: https://git.openjdk.java.net/jdk/pull/1666/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1666&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8196090
  Stats: 68 lines in 5 files changed: 33 ins; 11 del; 24 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1666.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1666/head:pull/1666

PR: https://git.openjdk.java.net/jdk/pull/1666