docs / rustpkg: Document `rustpkg test` more

Talk about `rustpkg test` in the tutorial, and update its usage message.
auto merge of #9738 : catamorphism/rust/rustpkg-test-docs-need-to-submit, r=alexcrichton

r? anybody Talk about `rustpkg test` in the tutorial, and update its usage message.

@steveklabnik, it would be great if you could look this over :-)