[IZPACK-1154] - Fix for set attribute of Combo fields and Tests

This is to address: http://jira.codehaus.org/browse/IZPACK-1154
1. SimpleChoiceReader has been modified so that combo boxes set attribute works corrrectly
2. Fixed GUI tests for the UserInputPanelTest, just same update to bring it up to speed with the current installer. Also commented out the test for the "processor" in combo fields. This should be re-added if an issue about this is raised and must be fixed.