How would the end buyer know if the tests were genuine? The vendors could easily be paying for good results since there’s zero transparency or ability to check.

Absolutely. For those testing communities too, reputation / track record is everything. And since a lot (but, sadly, still a minority) of customers will also reagent the product themselves - a lot of them in countries, especially in the EU, where such a test is free and anonymous thanks to non-profits or even in some case government backed services - any divergence is in theory quick to appear and would cast doubt on their reputation. But that's only the theory, and as you say it's far from a perfect system.

The community I mentioned (I don't know if it's still active) used to ask vendors for samples to test; but also ordered product from those same vendors (and others) as "fake" customers in order to make sure the product actually being sold is the one they tested. They also had donation runs with the community at large (both to pay for the tested products and to remunerate themselves).

They could also just send in pure / accurately labeled stuff in for testing, unless the testing company is ordering stuff at random from an alias / PO Box or something to prevent sellers from masking their true products ingredients.

Tech companies get busted for sending golden hardware samples to review sites, the truth eventually comes out.

Although no one dies if their video card is slower than advertised.

See also Volkswagen.

Exactly what I was thinking but I didn't want to drag in an outside subject without.

I think that just points back to reputation again.