Decouple proj and proj-sys tests

Once again, the fact that we run the same test suite for `proj` and `proj-sys` is causing problems: #230 can't be merged because it requires the `proj` tests to pass, but that's blocked on #228, which is blocked on #230, so we now have to decide whether we want to manually merge #230 and push a release. Is there some sensible way of doing this?

Start by reviewing how the `proj` and `proj-sys` test suites are run, along with the dependency chain between #228, #230, and #231. Determine a way for the suites and related checks to proceed independently; done means the blocking cycle is removed and releases no longer require manual merging.

Written by the indexing model from the issue text.