It should be input first, output second, like with all other APIs. I
remember I was trying something else here, but that didn't really make
sense in the end. Also took that opportunity to get rid of one
std::string.
The original signature is a deprecated alias to the new one and will be
removed in a future release.
It doesn't really work for tests that depend on more than one plugin
(because there i would need to handle all combinations, somehow), but it
does the job when the end user has such use case.