A way to test schedule reminders
It might help to differentiate types of testing, e.g.


(User-oriented) Click-to-test/in-app test - This is the type of test supported in the CiviMail UI (which sends an example mailing). As dlobo points out, this type of test was not originally part of the scheduled reminder system, but it would be a great addition.
(Developer-oriented) Unit testing - For developers who work on the code, there's a series of automated tests in ActionScheduleTest which simulate a dozen configurations/schedules.
(Sysadmin-oriented) Adhoc testing - At the low-end, one might simply create a fake membership record and set the expiration date to today. At the high-end, one might replicate a site in an isolated environment (eg a local virtual machine), redirect email to disk (CIVICRM_MAIL_LOG), and manipulate the clock in the virtual machine. (This may require a deep understanding of the system as well as a lot of time/patience/discipline.)