Add support for examples and external tests

This add support for `examples/*.rs` being built (as normal bin crates) during `cargo test`,
and `src/test.rs` and `tests/*.rs` being built and run (as test crates) during `cargo test`.
It happened [here](https://github.com/o11c/cargo/commit/508599b1c00770cd097326587d93608d5cdd294f#diff-f4550ee862e63b97b52a09a7f644ac7bR42). I don't see any indication why. Maybe someone at @tilde-engineering remembers?