This is an interesting article I found on the internet, describing the diffferences between lappin and testing and going through how they runs tests with the chassis, engine, and tires. Might not apply to the regular budget driver who cannot afford different axles, torsion bars, etc, but there are some details that I thought would be interesting to try once I run at consistent pace.
Tire testing can get very expensive too 
I like this article a lot. I don’t know that I’d have the patience for testing but he does break it down well. He emphasizes that most should be “lapping”.
“For a lapping day, the toolbox should be simple and light. Bring an air pressure gauge, chain lube, and fuel. While that might be bit of an exaggeration, it’s not far off. The setup on the chassis should be the ‘baseline’ setup the factory advises you to begin with, and the engine should be ran a bit on the conservative side (for example, a bit richer and/or lower RPM than race-recommended). The idea is to pound 100+ consistent laps ~not caring about the lap times, just the fact the driver can hit his/her marks every time. This is not testing. This is learning to drive a kart near its limit of control as consistently as possible.”
I agreed with that part too. You can’t do testing and make changes to the kart in setup until you are able to consistently drive your kart at the limit with the baseline setup. Once you do that, then you can start making changes and see if that improves times. I’m sure that doesn’t just apply to karting, but car racing as well