How can I set the output of now() in MySQL?

We want to run automated tests on MySQL stored procedures and functions wher the tests compare actual and expected values.

The trouble is, even if we run a fixture script before each test, most of our routines can return different results given the same inputs- we do a lot of datetime stuff that relies on now. (now is the built-in function for returning the current datetime.)

How can we "freeze" what now returns in our test database?

The best solution I can think of is to replace all calls to now to calls to a custom function. In our prod database, this custom function will just call the built-in now, and in our dev database it will return a static date.

Is there an easier way, such as something like SET global.CURRENT_TIME = '2015-01-01 00:00:00';?