Run tests for single Rust module

In Tor, we currently have the ability to run tests for a single C module (or even a single unit test). As specified in doc/HACKING/WritingTests, running tests for the cell format module (for example) can be done via ./src/test/test cellfmt/..

Rust modules should have a similar option. Currently 'cargo test' can be run within a single Rust module, but this will not link against C modules. It would be good to be able to do this and retain the ability to test a single Rust module. Also, it would be nice to make this similar to running single C module tests, to minimize developer confusion.