De-duplicate some unit test paramatrization

[dstansby]

This removes a lot of duplication in parametrizing the unit testing, by extracting a common decorator into a new file. I had to move the tests to a new sub-folder to get this to work, I hope that's okay?

[keewis]

with #7799 I believe we can keep things within a single test file: create and append the unit info instance to unit_libs within a try-except block (so import errors cause only these tests to be skipped), and skip the entire file if unit_libs is empty.

In principle, though, this idea does sound good to me, as well.

[dstansby]

Is keeping things in a single file a deliberate design choice? Personally from what I can see splitting up into separate files makes sense, given the original file is already so long.

[max-sixty]

(I'm going through outstanding PRs)

This does look like a nice simplification. Should we try and resolve conflicts, decide on the single file, and merge?

[dstansby]

I don't have time to finish this off now, but someone else is more than welcome to run with it 😄

[dstansby]

Closing since I don't have time to work on this any more