Support for --serve-uri instead of --pub-serve

Currently, it's not possible to run tests against anything but `localhost` because of a limitation: --pub-serve option only accepts the port number instead of a `Uri`.

Container based development could definitely benefit from this.

`pub serve` also supports `port` and `hostname` options, it makes sense to me that `package:test` should too.

Start by locating the package:test handling for the --pub-serve option and any CLI tests covering pub serve arguments. Check how the option currently accepts a port, then determine the expected URI, port, and hostname forms; done means tests can target a non-localhost address, including container-based development.

Written by the indexing model from the issue text.