Add `test` command to support running tests

[ithinkicancode]

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!