Document how to run unit tests

[spastorino]

If you want to run thin_vec unit tests you may try to do something like ...

./x.py test compiler/rustc_data_structures/src/thin_vec/tests.rs

But that's not possible.

The right way is ...

./x.py test compiler/rustc_data_structures/ --test-args thin_vec

We should add this information to https://rustc-dev-guide.rust-lang.org/tests/running.html and possibly discuss a bit about why unit tests are not supported using a single file. Maybe we should also support that through x.py.