Yo Coach! #1.You must fill the DWV prior to the inspectors arrival and it must hold for at least 15 minutes.#2.The fitting you are referring to is a test TEE,to get ready for the DWV test you must isolate your new system where you tie into the cast-iron in basement and where the vent ties back into the existing DWV.
If I recall you cut into 4"c.i. stack w/ a 4x3 wye you should install the 3" test tee coming out of the wye,then you need to buy the inflateable 3"combo test balloon+clean-out,they are sold at regular pbg supply house,with this fitting and test baloon you will not impact your existing DWV.
Now where you tie the vent from your new bath to the existing vent you also need to install a 1.5" test tee,this is where you can fill the new DWV with water for the pressure test.Personally I would put a test balloon in the 1.5" test tee and fill the new DWV with 5psi of air,this must also hold for 15 minutes,also do not forget to cap or plug your new fixtures for the test.