Adding org.codehaus.groovy.transform.powerassert.ValueRecorder

The compiler used to compile src/test/java/**/*.groovy seems to produce
a reference to org.codehaus.groovy.transform.powerassert.ValueRecorder,
while our Groovy runtime appears to contain
org.codehaus.groovy.runtime.powerassert.ValueRecorder.

I suppose the proper fix is to somehow run Groovy 1.8 compiler,
but this fix adds the necessary code to the classpath.