Ensure that snapshots compile

[liamnichols]

It was noted in #55 that there are some compiler errors with some of the snapshots that went unnoticed (due to more edge-case configurations).

It would be beneficial to ensure that the snapshots compile as part of the CI checks to catch this kind of thing earlier on.

Compiling a single file for macOS should be a relatively straightforward swift cli invocation. We might even be able to call it from the test suite itself rather than some other kind of script.