Is it ok to use push-in connectors for testing?
I don't believe you're referring to the push-in terminals on the receptacle, but push in connectors like these:  



In my experience, this type of push in connector is a safer bet than  wire nuts for someone that isn't practiced at using wire nuts.  I have not seen any real problems with these, and they are very easy to use.  

However, I would not use them for testing.  You have to pull and twist them to remove them, and it dings up the conductor a little bit.  

To simply cap single wires, insulating the stripped bit at the end for safety, wire nuts are fine for anyone - while it takes a little skill to get a good splice with wire nuts, anyone can cap a single wire - as long as the wire nut isn't too big.  

For testing when you may have to unmake and remake splices, lever nuts are best.  They are easy to use and easy to remove.