Just don't sign off someone elses work! Your name's on the certificate - you're responsible. Even if you've done all the tests, do ypu know where the cables have been routed? How many lovely diagonal runs are there? Have you checked all connections. Are there any illegal hidden junction boxes? Do the cable sizes take account of thermal insulation? How can you tell? On the sayso of some amateur?! When you sign that certificate you're saying that the installation (not just the function), in your opinion, complies completely with BS7671. The comeback's on you!
The only certificate that you can legitimately offer is an EICR.
Pete