While implementing the conical gradient ( flutter/engine#40236 ) I noticed there was no infrastructure for doing a golden image test with the result to make sure it doesn't get broken. Gradient tests can be flakey but we should have some guarantee that the gradients don't break.

design doc: https://docs.google.com/document/d/1_Ckq2Enl0PTAy987vDgcnDswe8PMoEFGwWbfB6aau2s/edit?usp=sharing