Add integration tests

[ericphanson]

I've been making some changes and was worried about introducing issues that cause failures to package CI. So I thought I'd add some tests. Here I've added Oceananigans, cc @giordano.

[giordano]

Yay! My only concern for you is that this will add a bit of precompilation latency for all the tests, but at least it's a real world large-ish codebase with many interesting corner cases.

[giordano]

Good to strictly fix the version, because I'm slowly but steadily going through all the modules to fix everything 😁

[ericphanson]

I've put them in a separate CI workflow, and just pushed a commit to parallelize over packages (though there's only 1 currently) so hopefully it's not too bad. My plan is to not run them when testing locally since failures here are unexpected (our normal tests should catch most things) but they'll run in PRs to catch issues before merge (hopefully).

[giordano]

Ok, it takes 3 minutes in total starting from scratch, it's not too bad after all. CUDA (which depends on DataFrames, which takes a while) is a pkgextension nowadays in Oceananigans.