Port the test suite to Rust

For the test suite, I think we should leave tests/api.c in C, since those are the C API tests.

But all the rest could be ported to Rust very nicely. At least, Rust would run the tests in parallel, which should hopefully speed up test runs.

I found this crate that generates test functions based on filenames. Maybe that would be useful to replace some of the stuff in test-utils.c?