@ Rob Ginger
On modern aircraft, the nose gear is kept in place by overcentre lock held in place with large springs ("bungees") and, if hydraulics are active, by a small hydraulic piston helping the overcentre lock and a much larger hydraulic piston pushing the nose wheel back down. Also, depending on the aircraft type, there will be either a mechanical restriction on the gear lever in the cockpit to stop you moving it to UP on the ground, or electronics to inhibit the signals from the landing gear lever to the gear.
You can further enhance safety by locking the overcentre lock with a steel pin. These pins are the same pins used during maintenance tows, but, for gear retraction tests, there is usually a requirement for a visual inspection of the pins to make sure they look perfect. There may be a requirement to send the pins away after gear tests have been done.
Regarding weight. Unfortunately, the aircraft weight is not much of an issue with the nose gear. The nose gear has wheels which have no brakes*. Some nose gear struts, however, are sloped backwards slightly (not vertical) or the wheels are castered, so the nose wheels have to scribe an arc "through" the tarmac to move forward (to the retracted position). Unfortunately, the hydraulics are usually strong enough to push the aircraft upwards.
Now, carrying out a test like the engineers did removes some of the safeguards (hydraulics now assist the retraction) and the gear lever inhibit is being removed. Basically, the safety pin, the bungee springs (which will be overcome by hydraulics) and some aircraft weight are the only things stopping the nose gear retracting. The pin is the primary safety device here and perfectly suited to the job, but, as has been rumoured, it was removed by a 2nd party. Note that there needs to be a sequencing of hydraulics. The overcentre lock has to be activated first by hydraulics, then the main retraction hydraulic piston/s can start moving the gear.
I'm just curious to know why door tests on the A330 can't be done without gear lever movement. On a 747-400, for example, you can manually release the door, then pressurise the #1 hydraulics system... The door will go back up.
* chocks may have been an impediment to wheel rotation