What do you mean if you like testing your own nitrox? That's not optional to me, if you insisted on checking your own nitrox they will pull the gear onboard between each dive correct?
I was on the Vortex a few weeks ago, same system.
While they'll do everything to help you test your own tanks if you REALLY want to, practically it's impossible to do it.
Your tanks (& BCD) are put in the pangas while the panga is on the boat, then the panga is put out in the water from a crane on the upper deck. The tanks then stay on the pangas all day and they're filled out there between dives with a long hose.
If you want to test yourself, you have two options: have the crew haul your tanks back to the boat after filling so you can test them on the boat, or test them out on the pangas yourself.
In practice, neither of those is easy. There's usually a swell in the sea and just getting yourself in and out of the panga is hard enough and usually requires someone on the boat AND someone on the panga helping you. Asking the crew to do the same while carrying a tank, and doing it two or three times a day, is unreasonable because there's a high chance someone will get injured. Testing in situ on the panga is almost impossible too. Holding an O2 sensor against the first stage when the boat is being tossed around, and expecting to get a stable reading just ain't going to happen.
I tested my tanks on the first night and found them to be exactly what they'd told me they were. I then figured the dive guides were far more experienced and competent than I am ever going to be, and I might as well trust them with my gas since everything else I was trusting them with had a far greater chance of killing me if they got it wrong.
I've not been on the Agressor, but the dive crew on the Vortex was in a different league to any other dive guide/master/professional that I've ever experienced, and I had no problem trusting them with every aspect of my dive. Usually I won't let anyone even touch my fins