BTW, I was once also in desperate need to override the filetest operators... so I'm pretty sure it cannot be achieved with the current implementation — but also see the patch mentioned in footnote 2 in the post I linked to.

I personally believe that in that case one may use source filters as an extreme resource, but... they're generally deprecated and I presume even more so in the context of tests, which is most probably the one in which need of overriding and mocking functions is arising... (Was it yours too?)