What is NH overspeed test and how it is preformed on DHC Q400 aircraft?
The same statement is made here in a document describing the PW150 turboprop engine, which powers the de Haviland Canada, now Bombardier,  Dash-8. 

It gives a little more detail in section 12.23.14.4, NH overspeed protection.

It says:


  The PW150A Powerplant has an independent overspeed (O/S) protection circuitry (dual chan- nel) built into the FADEC which has the capability to cut off the fuel flow through the Fuel Shutoff Solenoid. Independent NH signals (from the FADEC) are used by the O/S circuitry. A fuel shutoff command is issued when the measured frequency of the NH input signals exceed a pre-pro- grammed threshold value of 108%. The O/S Protection circuitry is exercised on normal shut- downs by the FADEC.


So, it would appear that, when testing the overspeed protection in a normal shutdown, they do not actually allow the shaft speed to increase to 108%, but rather they electronically alter the speed sensor frequency signal, so it appears like the shaft speed is greater than 108%, which then cuts off the fuel flow.

NH is the term used to describe the high pressure shaft speed, measured in units such RPM.