fix(mock): Timer.isActive should be false after running callback

For non-periodic timers, isActive should be false after running the timer function.

https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/dart-async.Timer#id_isActive

Author: chirayuk

lib/mock/zone.dart

@@ -152,6 +152,7 @@ void clockTick({int days: 0,
         if (spec.duration <= Duration.ZERO) {
           microLeap();
           spec.fn();
+          spec.isActive = false;
         } else {
           remainingTimers.add(spec);
         }