Best practice involves encapsulating your code. To put that another way, if you have written a
triggerHandler Class, you should create a corresponding
triggerHandler_test Class should test the
triggerHandler independently of the trigger itself. This is how you can encapsulate your trigger handler code and segregate it from the trigger.
Chances are, a trigger handler is going to have a list or a map (or both) as an input parameter. Your unit test will need to setup that test data. Naturally you will want that test data to look like it came from a trigger.