runtest.sh takes one argument that is the browser implementation swt was built
against. So for examples:
	runtest.sh xulrunner

(xulrunner) is the default

The test should load http://www.eclipse.org to a window. This window does not
seem to close properly so just pkill -9 java if you don't have anything else
java running.

You should test both seamonkey and xulrunner, like this:
USE="seamonkey -xulrunner" emerge swt
runtest.sh seamonkey
pkill -9 java
USE="xulrunner -seamonkey" emerge swt
runtest.sh xulrunner
pkill -9 java

(if you put both in USE, it builds against xulrunner)