Can't test if using duration property.

[nakuzm]

Describe the bug
If I use duration property, then I have exception testing my widget that uses Flushbar for displaying error message.
Timer needs to be canceled on dispose.

The following assertion was thrown running a test:
A Timer is still pending even after the widget tree was disposed.
'package:flutter_test/src/binding.dart':
Failed assertion: line 1241 pos 12: '!timersPending'