You don't actually have to do a 170a flight test for your CPL test. Its a rip off done by most schools who will proberly charge you a test fee for the unrequired flight test.
The 170a for a CPL is a paper work exercise which the person who is allowed by the caa to sign them checks all the records and makes sure that everything is right for you going to test.
The 170a for IR is a flight test as well as the paper work check.