Make separator_detector_spec idiomatic

* Make it closer in style to our other specs

This is pretty much just because I wanted to see what the difference is. There's a marked change in style from our other specs in here and I wanted to get a feel for how it would look in our "usual" RSpec-let-parameterised style and whether we're intentionally moving away from that. Not saying either is "better", but just wanted to see. Opinion time :dash: