Avoid run DefinitionsCollectorBackgroundTask on AWT in tests

idea/src/org/jetbrains/kotlin/idea/script/AsyncScriptDefinitionsContributor.kt

@@ -72,6 +72,8 @@ abstract class AsyncScriptDefinitionsContributor(protected val project: Project)
             }
         }
 
+        override fun isHeadless(): Boolean = false
+
         override fun run(indicator: ProgressIndicator) {
             while (true) {
                 if (indicator.isCanceled || !shouldStartNewUpdate) {