RFR: 8281988: Create a regression test for JDK-4618767

[Manukumar V S]

Create a regression test for [JDK-4618767](https://bugs.openjdk.java.net/browse/JDK-4618767)

Issue identified in [JDK-4618767](https://bugs.openjdk.java.net/browse/JDK-4618767):
Typing a letter while a JList has focus now makes the selection jump to the first/next node/item whose text starts with that letter even though that letter is accompanied by modifier keys such as ALT or CTRL.

Fix:
Only enable JList letter navigation when the user
doesn't press any modifier keys such as ALT or CTRL.

Testing:
I have verified this test with JDK 1.4.0 and JDK 1.4.1 .
The issue is reproducible using the test with JDK 1.4.0 , where the bug was originally reported and the test passed in JDK 1.4.1 where the issue was fixed.
I have tested it in Linux, Mac and Windows platforms multiple times and it passed everywhere.

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Commit messages:
 - 8281988: Create a regression test for JDK-4618767

Changes: https://git.openjdk.java.net/jdk/pull/7496/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7496&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281988
  Stats: 166 lines in 1 file changed: 166 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7496.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7496/head:pull/7496

PR: https://git.openjdk.java.net/jdk/pull/7496