Hi peeples. Sooner or later online tests will come with a 'pause' button for parents juggling children. That doesn't help you now though, does it?
The age gap between my children is so big that my pre-schoolers never had a sibling close in age, so I needed to encourage them to be happy to play alone.
When my kids were little and I needed time and space to complete projects, I'd have a big blackboard they could draw on (with lots of chalk on hand ... some where they could reach them and some that I could easily pass to them). Sometimes I would draw shapes for them to colour and try to copy (before I began).
One of the things I loved was the invention of the pre-schooler's computer. The type that had buttons and would play them songs or make different noises. I would suggest that they work on their 'computer' to help me get my work done faster. If the noise will distract you though, that's probably not a good idea - but you could keep an eye out for one at a garage/yard sale and give it a try. It needs to be basic enough for them to be able to use it without constant help from you.
A pile of blocks could keep them busy for ages, trying to balance them all on top of each other. If they could do it by themselves, I rewarded them with a slice of apple. Knock them down and do it again for another slice.
One of my most successful strategies was to build them a cubby house out of blankets dangled over chairs (not the type that will fall over on them) - and then suggest they find their toys to take into the cubby. Before I needed them to entertain themselves I'd hide toys in different places - & an unexpected balloon for them to discover and play with.- and make myself a list so I could quickly point them in the right direction if needed. "Have a look under your bed. You might find teddy there." And when they're getting disgruntled again, "I think you should look under all the pillows."
When they had four or five or however many toys it took before they lost interest in the search, I'd pass a plate of chopped up fruit (concealed on my desk) and say, "Now go and share nicely with your toys". Then the fun of the picnic began. "Make sure they have good manners and share nicely."
I found I could put myself on automatic pilot and not lose concentration, and it worked well for me. Of course I wasn't doing tests though. It might take a few practise runs to get the process smooth enough for you to complete your next test uninterrupted.
Good luck with it. :)