proper test (amending 32f9e75c92e9);

--- a/src/Pure/Tools/build_process.scala	Wed Mar 08 22:40:47 2023 +0100
+++ b/src/Pure/Tools/build_process.scala	Wed Mar 08 22:42:21 2023 +0100
@@ -145,7 +145,7 @@
       build_heap || Sessions.is_pure(name) ||
       sessions.valuesIterator.exists(_.ancestors.contains(name))
 
-    def worker_active: Boolean = max_jobs > 1
+    def worker_active: Boolean = max_jobs > 0
   }