I've heard that the justification for this is that allowing testing would let illegal drug manufacturers to become more effective at making drugs.

Of course allowing testing could save a lot of lives too, but that's apparently never been the true motivation of the War on Drugs.

If you happen to know: Is it the performing the test itself that's illegal, or is it "merely" that to perform a test for someone you'd have to take possession of the substance, however briefly? (Bad for both harm reduction and liberty either way, I'm just curious which it is.)

But the tools themselves are just paraphernalia and legal?

I guess maybe this is explicitly making them not useful as probable cause, and also making their use a defense or legal.