Add `test` command to support running tests

Currently there is no `test` command in `cargo-component`. The standard `cargo test` doesn't work because the standard tooling has no visibility to `bindings`, `exports` or any generated structs and enums. I'd think it's a good devX that devs can follow the same workflow just like when writing regular Rust apps. Please point me to the right direction if I miss any docs wrt testing!