You gotta have Mode C on anyway for a TCAS test. Also for the test (at least with Collins on Bae146) you pump a/c up to 10,000' then use the ramp test set to simulate 12700',12800', 7300', 7200', and on the nose at 10,000'. This is to test the ABV/BLW target rejection and test TA/RA indications.
This is from memory but think alts are correct. Also A/C is simulated airborne and Radalt is above 2500' so Mode S will think its flying and will respond to ATC interrogations. Inside hangar is usually enough but there is possibility of it showing up on ATC screens and other aircraft TCAS.