Adds a simple JJS test to verify that enums compile.

TODO: improve this test.

http://gwt-code-reviews.appspot.com/640803/show
Review by: spoon


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@8290 8db76d5a-ed1c-0410-87a9-c151d255dfc7