Is it safe to test new digitiser before reassembly?

no issue to test but as with all devices make sure you connect everything before connecting the battery. If using a device with a Touch ID home button such as iPad mini 4 that is ok to attach after the battery because the connecting cable sits over the battery connector. Before you seal the iPad mini make sure the new digitiser has got the connections at the bottom taped off with electrical or caption tape or else they will make ghosting happen when the touchscreen shorts against the case. Also make sure you follow the fixit guide to make sure the digitiser cable is placed in the iPad correctly.