Pretty sure the answer is when people try to actively hinder half a loaf because it's not the full loaf they wanted.
See also: The Affordable Care Act.
Saying" I advocate for policy X" can never be a purity test.
Saying "I oppose X/2 because it isn't X" always will be.
Not sure how it's a slur tbh but maybe you've seen people using it differently.
Yeah, like 100% of the time people actually use it.
The actual conversations look like:
A: I want a candidate who supports policy X. It is a key policy for me.
B: What about policy X/2?
A: Whatever, that's not X, we should demand X.
B: Purity test!
In other words, the actual argument here is whether X is an important enough policy that demanding the candidate support it is reasonable, or whether it's not practical or political and we should settle for X/2. "Purity test" is used to suggest the people who prioritize X are being unreasonable, by the people who don't prioritize X. That's what makes it a slur.