he circuit is quite simple. Source of gate voltage is switched between two options by a relay. The relay is driven by signal generator (you can use anything like Arduino for the purpose...).
One source is an adjustable low positive voltage. It is used to test gate threshold voltage of the mosfet under torture.
The other source is the large negative voltage, which is to cause biasing.
The mosfet's drain-source current is monitored by observing brightness of an LED. I am using a power LED and a Li-ion cell for this. Indicator LED will also do, but it doesn't allow to test for high currents.
The generator is set up so that 90% of the time, the large negative voltage is applied to gate, and periodically it is briefly switched to test voltage. To measure gate threshold voltage, I vary the test voltage (using that long linear pot) to the point I start seeing the LED blink.
The nice thing about this setup is that I can monitor how threshold voltage of the mosfet changes, in real time.