replace
environment 'MOD_CLASSES', 'dummy' // Needed to work around FG limitation, FG will replace this!
in build.gradle under
project(':forge') > patcher > runs > forge_test_client
with
environment 'MOD_CLASSES', "${project.file('build/resources/test').canonicalPath};${project.file('build/classes/java/test').canonicalPath}"
(maybe you have to replace ; with : )
and start the forge forge test client