Move tests to standalone binary

[jhpratt]

Fixes #13

Currently a linker failure for the iOS build. I am unable to reproduce that failure locally (using Linux).

[Freaky]

The macOS linker failure looks identical to rust-lang/rust#105167 - I think the fix would be to bump the Rust version there, or request an older macOS image.

The sleeps are a bit disconcerting - are the thread counts reported by /proc actually potentially out of date, or is Linux just lazy about reaping LWPs?

[jhpratt]

Requesting an older image is a losing battle, as they eventually get turned off in favor of newer images. I can look into checking a higher Rust version in CI, but I'm not sure where to begin on that other than doing a binary search manually.

With regard to the sleeps — I have no idea why they're necessary. The tests pass on my Fedora machine without them, but it failed in CI.

My priorities are elsewhere in the ecosystem at the moment, so feel free to push on this branch and release when ready.