Improve tests

[thomcc]

Test coverage here is pretty abysmal. Filing a bug to record that it should be improved.

The stdlib has some tests for Arc functionality we could borrow in https://github.com/rust-lang/rust/blob/master/src/liballoc/sync/tests.rs, but a lot of them test things we don't really support like weak references.

They also don't test any of our bespoke artisanal smart pointers.

It would also be nice to run tests under loom once we actually have tests.

[Manishearth]

This would be great!