rustdoc: Use --crate-name with --test

This ensures that the name of the crate is set from the command line for tests
so the auto-injection of `extern crate <name>` in doc tests works correctly.
auto merge of #16157 : alexcrichton/rust/rustdoc-tests, r=huonw

This ensures that the name of the crate is set from the command line for tests
so the auto-injection of `extern crate <name>` in doc tests works correctly.