Ensure crates are tested after packaged

The `target` directory was erroneously being used from the crate root rather
than the root of the package directory itself, leading to packages not actually
testing what's precisely being published.
auto merge of #944 : alexcrichton/cargo/test-some-packages, r=brson

The `target` directory was erroneously being used from the crate root rather
than the root of the package directory itself, leading to packages not actually
testing what's precisely being published.