I worked with many old timers , a leak test was to light a match, and run the flame around the fittings .
What is needed to complete the task here is a MINOR , task not , to cause leaks . Just turn off the shut off , what ever it maybe , cap or plug what is remaining . Leave the shutoff closed , soap test it and carry on . I have no idea how most of the responders to the question would handle what I ran into on a daily routine day . I worked daily with CFR 192 , 195 , NFPA 58 , 59 and am a Certified Training Instructor in all of these. Don’t make an issue , out of every task . Do it safely and correct the 1st time because if you don’t how do you have the time or knowledge to repair it again the 2nd time . Be safe , every task has been your 1st task at one time or another in your life .