Yes, they are interchangeable. In "swim test", the noun "swim" is re-purposed as an adjective to modify "test". It is common to do this. Other examples: "algebra test", "leather shoe", "dog bed" (where a dog rests), "fountain pen", "dining room table", "factory floor", "cruise ship", "fruit beverage", "star student", etc. Since "swimming" is the present participle of a verb, it can be freely used as either a noun or adjective. So in "swimming test", you can view "swimming" either way.

Either one can be used to describe a procedure for assessing a person's swimming ability. An organization would normally only use one or the other, but if you use the one they don't use no one would misunderstand you.