The hack that Curtis mentioned about
disabling JET builder is valid just for those that doesn't need to modify
the Test related plugins (especially *.test.manual, *.java, *.http). If
you need to modify the JET templates and regen then you will need
to enable the JET builder (so you'll be back to square one again).
I opened a defect against EMF codegen
(suspecting that the real problem is there):