Pair of single test lead wires to solder type barrel plug: how do I strain relief the connection?
The spikes in the connector are intended to be crimped overtop of the cable insulation to strain relieve the connector. The holes are to accept the wire and solder. 



The boot is not useful for strain relief as is, but it will prevent sharp bends that can cause the cable to fail. 

This kind of DIY connector cannot be compared to a molded connector in terms of strain relief. You could get some of the benefit by filling the boot with hot glue before assembly but it might take a couple tries to get it right, and it cannot be fixed most likely if it breaks.