Include cuda_bindings Cython tests in main cuda_bindings test suite

[mdboom]

The Cython tests in the cuda_bindings test suite currently require a separate build step, otherwise they don't run as part of the main cuda_bindings test suite.

This makes them easy to forget to run, and, as in my case, I didn't even know they existed.

Instead, we can do the build step as part of a test fixture. This pattern is already used in cybind so it can test Cython-compiled things.

Opening an issue in case there is something I'm missing as to why this can't be done.

[mdboom]

It seems like the main reason for the division is the lack of compiler on all of the testing CI nodes. So we need to maintain somewhat of a division, but I think it still might be worthwhile to combine things more for local development.