<AWT Dev> RFR: 8233564: [TESTBUG] MouseComboBoxTest.java is failing

[Prasanta Sadhukhan]

Test was problemlisted due to it failing repeatedly on macos in automated nightly testing citing "java.awt.IllegalComponentStateException: component must be showing on the screen to determine its location "
Added delay after the setVisible() is called to make sure enough time is given for frame to be visible, which is in sync with what we do for similar other tests. 
Also, made the frame to center of screen and increase the autoDelay time to 100ms which is also same as done in similar to other tests.
Several iterations of the test works fine in all platforms after this modification.

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Commit messages:
 - 8233564: [TESTBUG] MouseComboBoxTest.java is failing

Changes: https://git.openjdk.java.net/jdk/pull/3456/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3456&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8233564
  Stats: 6 lines in 2 files changed: 3 ins; 1 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3456.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3456/head:pull/3456

PR: https://git.openjdk.java.net/jdk/pull/3456