Allows GWTTestCase#getModuleName() to return null to indicate it should be run as a normal JUnit tests case even though it extends GWTTestCase. See the issue for the rationale.

Patch by: bruce
Review by: scottb, amitmanjhi
Issue: 3772



git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@5617 8db76d5a-ed1c-0410-87a9-c151d255dfc7