There are a whole range of tests from helmets dropped onto different shaped anvils, to using different forces in he drop. Some test straps, and retention, others don't
Equally some allow manufacturers to provide batches specifically for testing, so you can get an approval for he helmet from ten testing of the prototype. Others such as Snell test off the shelf helmets - the same as you or I will wear
The problem with Snell is that companies have to pay (IIRC) and hence whilst EN1078, CPSC etc are required for sale, Snell is not, so only Specialized really invest. Personally I would depend on Snell, but that is entirely my opinion and no more or less valid than anyone else's
One interesting point is that ten years ago Snell B95 was considered the best but there are no longer any helmets that meet this
Technically some tests are better than others......
Testing on a kerb shaped anvil is a better predictor of an impact with a kerb than testing on a round anvil for instance. Equally testing with a greater force is going to prove more strength in the design and better protection in an impact
However as there are tricks such as taping the helmet in place he true answers will never be known as to whether the tests reflect real life.
Anecdotally EN 1078 is no longer accepted in the US for racing or Triathlons and Snell B 90 is the toughest to pass.
If you are really sad and want to research what is involved look at
Bicycle Helmet Safety Institute It is a US site, but their description of the various tests is excellent