Create a regression test for the JTree usecase of JDK-4618767

Issue identified in JDK-4618767:
Typing a letter while a JTree 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 JTree letter navigation when the user doesn't press any modifier keys such as ALT or CTRL.

Testing:
Java 1.4.0 -> Test Failed.
$ ./j2sdk1.4.0/bin/java JTreeOld
LookAndFeel: javax.swing.plaf.metal.MetalLookAndFeel
OS = windows nt (unknown)
java.lang.RuntimeException: Test Failed, expected colors, but got food
        at JTreeOld.doTest(JTreeOld.java:166)
        at JTreeOld.main(JTreeOld.java:32)
Exception in thread "main"

Java 1.4.1 -> Test Passed.
$ ./j2sdk1.4.1/bin/java JTreeOld
LookAndFeel: javax.swing.plaf.metal.MetalLookAndFeel
OS = windows nt (unknown)
Test passed javax.swing.plaf.metal.MetalLookAndFeel
LookAndFeel: com.sun.java.swing.plaf.motif.MotifLookAndFeel
OS = windows nt (unknown)
Test passed com.sun.java.swing.plaf.motif.MotifLookAndFeel
LookAndFeel: com.sun.java.swing.plaf.windows.WindowsLookAndFeel
OS = windows nt (unknown)
Test passed com.sun.java.swing.plaf.windows.WindowsLookAndFeel