Great minds think a-like, I had to make a long cord, I could attach to the appliance, and then switch out the plug ins, Used a heavy 6-3 so cord. I have a four lead plug , a three lead three straight, and the three lead with one ninety, (L) that I can wire nut to the end.
I have some comercial accounts that have lots of other appliances infront of there one and only 220 outlet. One has the three straight outlet the other has a 4 wire outlet I'm not going to move their entire inventory around in the stock room or shop to plug in their 3 foot cord.
As you say FOR TEST PURPOSE only..
Worked great for me at a motel, and customer wanted me to fix or look at another dryer, but only had the one outlet.
I use what you have made in the post above but I put a 4 plug in on a three L box.
My incoming box in shop (one and only) is a four, now I can test at least a customers dryer in shop with a three wire with L with out R & R the leads.