I think you’ll struggle to find any exhaustive test results on MX helmets.
Go with a brand you’ve heard of, a proper and comfy fit, oh, and of course a colour that matches your riding gear
If you want to race, you’ll need it to have an ACU sticker on it. This has no bearing on the helmet quality; the importer just fills out a form and buys the stickers. (I think you can still get them on fleaBay as well)
Don’t go too silly on the price; if one subscribes to the established wisdom that a helmet should be thrown away after an impact, I would need a new one almost every ride. I did buy a new one before I went hooning around Morocco because I'd had many small offs with my existing helmet and knew I'd be riding a tad faster than normal.
For normall day to day riding, almost anything will do (I don't think I'd trust a Lidl own brand though)
Go to a shop that keeps a few different brands and try them on. Fit is most important.
I like HJC. They fit my (funny shaped) head well, are not too expensive and seem to hold up well in falls. The ventilation is pretty good and the linings are removable too which means you can give it a proper clean once in a while.