We could also disable the forked mode for tests to run them in IntelliJ IDEA. We must then set grails.project.fork.test = false in grails-app/conf/BuildConfig.groovy.
This is a solution to problems running grails tests from within IntelliJ failing when they run in the console correctly.
The BuildConfig.groovy approach to fixing this issue seems cleaner than the approach of copying the jar file every time. My environment was IntelliJ IDEA Version 13.1.2, Grails 2.3.7 and Java 1.7.0_40.