For testing the loop impedance function of the tester you simply need a designated  socket that is not protected by an RCD. The reason for this is because some loop testers have the facility to lock RCD's while carrying out their test. In this mode some testers are not as accurate.

This is because maximum Zs of RCD protected circuits is lower than circuits without RCD protection. Some manufacturers have considered this when designing their tester. I therefore recommend carrying out impedance tests with the 'D-Loc' function disabled so variations can easily be detected, and to do this you need a non-RCD socket.

It is important to place a  warning label next to the socket, to explain that it is not protected by an RCD, and that it is only to be used for meter accuracy checks. The reason for this is because regulation 411.3.3 of BS7671:2008 requires the label to be present.

For checking the RCD function of your tester, all you need is a 13A RCD adapter (like the one shown below). I recommend  you reserve the device purely for meter checks. The reason for this is because it will not be in operation like an in-service RCD, and therefore more likely to show up incorrect meter readings.