Is there a resource that lists certifications for all available helmets?
I think the reality is that the best you can hope for is a per-country, per-tester, per-standard or per-certification list. There are many bicycle helmet safety standards I don't believe there is any single worldwide source of bicycle helmet certification info.

When you're at the store trying the helmet on to see if it fits comfortably, look inside the helmet for a certification sticker.  Mine has a Snell sticker that says "B90A" and I can go to Snell's Helmet Certification List to search via the testing standard (B90A), make and model to find that they have tested that helmet to that standard.  (and if I want I can also look around for the details of that standard).

Or perhaps you're not trying the helmet on and are contemplating buying the helmet site unseen? That sounds likes a bad idea to me, I'd want to insist on trying on a helmet before buying to make sure it fits my particular head well.

Snell essentially has their own higher standards that exceed the standard US standards and has a very nicely searchable online database.  POC does not appear to use Snell because they don't list Snell on the POC for the other helmets and I can't find POC helmets on the Snell site.

The POC link you provided has some stuff with a European standard (EN), some with a "CPSC" and some with "ASTM".  CPSC is the US "Consumer Products Safety Commission" who certifies testing procedures and testing labs and publish recall information, but I don't believe they actually do helmet testing directly. ASTM seems to be a worldwide standards organization, who similarly to CPSC certifies testing procedures, not helmets.

It's quite likely that POC uses the same outside lab for testing all of their helmets to the standards, so it's worth looking at who certifies to those standards they do list.

By searching on ASTM's website for "bicycle helmet" and clicking the "Laboratory" refinement button, and they have a listing for Biokinetics and Associates and ICS Labs. Otherwise ASTM's site isn't really very useful...

Biokinetics has a client listing that includes Bell (one of the bigger helmet manufacturers), but not POC.  No product listing.

ICS doesn't appear to list their clients, and the Certified Products link is under construction.

Which all leads back to: look inside the helmet for the certification sticker.