They can be in more countries than just the Netherlands IIRC. And simple test-kits are available too - these aren't as good obviously, but can e.g. prove the absence or presence of certain (by)products.
Also, these tests are set up in a way that they are anonymous in a way that e.g. even if prosecutors could subpoena a test-location or lab to hand over administration, this administration won't contain any PII or even pseudonymous information.
I was under the impression that it's impossible to reliably test, e.g., cocaine for fentanyl, without destroying the drug and it doesn't take much of the latter narcotic to kill you.
Also, these tests are set up in a way that they are anonymous in a way that e.g. even if prosecutors could subpoena a test-location or lab to hand over administration, this administration won't contain any PII or even pseudonymous information.