Auto merge of #4931 - Manishearth:testname, r=matklad

Explicitly mention testname argument for cargo test/bench

The fact that `cargo test foo` works is totally non obvious. I suspect that 99% of the time
folks running `cargo help test` are looking for a way to only run a specific test, and
the current help message makes it seem like the way to do that is `cargo test --test foo`,
which is incorrect (it runs a specific test target).

This PR mentions it explicitly in the help message, first and foremost.