[IZPACK-1153] - Fixing GUI Target Panel Tests

This is to address: https://jira.codehaus.org/browse/IZPACK-1153
Fixes for TargetPanelTest.java

1. Fix testEmptyPath test so that we test for the "TargetPanel.warn" message. (See issue for details of why)
2. Fix testNotWritable test so that the correct root directory is used rather than the root of the tmp directory.
3. Add a question in warning form to heighten awareness of overwriting contents. (Notice how the target panel tries to convey this with the "TargetPanel.warning" key)
![question_warning](https://cloud.githubusercontent.com/assets/1176292/3895893/1c38eafc-2252-11e4-87b7-e0c090e4cc57.png)