Testing a custom Django template filter
The easiest way to test a template filter is to test it as a regular function. 
@register.filter decorator doesn't harm the underlying function, you can import the filter and use just it like if it is not decorated. This approach is useful for testing filter logic. 
If you want to write more integration-style test then you should create a django Template instance and check if the output is correct (as shown in Gabriel's answer).
I have a custom template filter I created under project/app/templatetags.
I want to add some regression tests for some bugs I just found. How would I go about doing so?