testing custom language: note on testing with Gradle

tutorials/writing_tests_for_plugins/tests_prerequisites.md

@@ -25,6 +25,13 @@ All we need to do is point the run configuration's working directory to the root
 -Didea.system.path=../test-system -Didea.home.path=../ -Didea.config.path=../test-config -Didea.test.group=ALL_EXCLUDE_DEFINED
 ```       
 
+If you're using Gradle, system properties must be passed to forked JVM via `build.gradle`:
+```
+test {
+  systemProperty "idea.home.path", "/path/to/community/"
+}
+```
+
 **Note**:
 
   >  Keep in mind that we have changed the _working directory_, so all the paths in tests extended from