duration() is specified to return -1 when none exists

testing/agent/src/main/java/sbt/FrameworkWrapper.java

@@ -151,7 +151,7 @@ class EventWrapper implements Event {
 	}
 
 	public long duration() {
-		return 0;  // Just return 0 as old event does not have duration.
+		return -1;  // Just return -1 as old event does not have duration.
 	}
 }
 
@@ -259,4 +259,4 @@ class RunnerWrapper implements Runner {
 	public String[] remoteArgs() {
 		return new String[0];  // Old framework does not support remoteArgs
 	}
-}
+}