USB 2.0 droop testing
I would like to jump in on the question.

For droop testing a specific a specific test fixture is used to simulate the Droop.

You need to load the port under test with the droop portion of the fixture, the droop will vary for USB2.0 and USB3.1 - 100mA/10uF for 2.0 and 150mA/10uF for 3.1

The limits for the Droop test are as follow:

USB 2.0 Bus-Powered hub 100 4.4

USB 2.0 Self-Powered hub    500 4.75

USB 3.0 Self-Powered hub    900 4.45

PC USB 2.0 Host system  500 4.75

PC USB 3.0 Host system  900 4.45

The load waveform will look something like this (Channel 1 is the Droop trigger, Channel 2 is the actual droop which is ~300mv in this picture) :