GWT test cleanup

Get gdx-tests-gwt compiling to JavaScript again.  I added some GWT stubs for NetApiTest, but ended up disabling it for now (I'm not sure how to fix its abuse of FileHandles).  Added some stubs to the GWT java.lang.Thread to actually implement the Uncaught Exception handler (so the RunnablePostTest will compile).

Cleaned up the index.html page, and made the test runner a bit more verbose (enabled the "console" too).

Let me know if you'd like me to try a different approach on any of the GWT stubs, or if anything looks like it needs a format pass.