May I ask you a couple of questions?
1. What is the real purpose of this test, and what is the expected outcome? If for example the SUT does not pass linerate, error free traffic for N days, are you expecting Juniper or anyone else to take this result and so what? I would think without RFC 2544 or similar test results, Juniper would not consider such a test valid, but maybe you do.
2. What is the reason for having the MX in the picture to start with - are they acting as the traffic generators somehow? For what you are asking of MX, why not instead use an EX as no-mac-learning on that product set does work. Just a suggestion.
Good luck. I do assume you have disabled any and all protocols and storm control, etc. and no mac-aging on the SUT, yes?