Fix packaging

[NathanReb]

I ran into some errors while testing out the repo before attempting the release, this should fix those:

1. Correctly allocate tests to their respective packages. Our main test suite now is affected to ppxlib so they aren't run when running something like dune build -p ppxlib-tools or opam install -t ppxlib-tools. This prevent some test dependencies to leak from ppxlib to ppxlib-tools.
2. Bump our dune lang version. Initially opam CI reported that we had to update dune lang to 2.9 but while solving 1, I actually ended up having to bump it to 3.8. This enabled some warnings that I guess where not part of the default build profile before and I ended up fixing those as well.

[NathanReb]

It seems we can't currently allocate cinaps stanzas to specific packages so I ended up also adding a test deps on cinaps in ppxlib-tools for now. I reported this upstream: ocaml/dune#11232.

If there's a better, easier fix, I'll of course patch this before the release!

[NathanReb]

Not sure I understand what's wrong with the bench build, are you any more familiar with it than I am @patricoferris ?

[NathanReb]

Ok the question rather was how did it even work before but I fixed it. It needed an explicitly declared dependency on the drivers/ folder. I guess bumping the dune lang changed dune's behaviour and it stopped copying those files to _build directory without any explicit dependency over them.

[patricoferris]

Thanks @NathanReb -- sorry for only getting around to this now, post holidays. This seems like a good clean-up to me

[NathanReb]

No worries @patricoferris ! I was on holidays myself as well!