Testing the brakes in flight may do something or nothing, depending on the aircraft and systems. Some brakes are completely disconnected in flight or bypass, others employ snubber systems internally which stop the wheels when the weight is off, or during retraction, etc. In aircraft with a wheel inbalance, applying gentle brake after takeoff sometimes reduces vibration.
So far as testing before landing, you're accomplishing little. A poorly maintained brake cylinder may be bypassing, and may require several applications to get a solid feel. In this case, applying pressure prior to landing is of benifit. Otherwise, if the system is properly maintained, then you shouldn't have an issue.
If one brake cylinder is weaker than the other, then "pumping" up both brakes by ensuring a solid feel may help prevent the aircraft pulling to one side on landing. Ensure if you're "pumping" up the brakes that you're not holding them, of course. If you're going to be landing in icing conditions, especially if you departed with wet wheels or brakes, applying brake may break ice which has formed over the caliper or pad area, and is sometimes benificial. Conversely, if you're departing such a location, allowing the wheel to spin and sling off moisture is also important, rather than stopping the wheel with the brake.