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.
I considered suggesting the `match`, but I figured a multi-line solution like that would not be as desired.

If we had rust-lang/rfcs#160 this would be a good candidate for `if let`:

```rust
if let Some(crate_name) = crate_name { krate.name = crate_name; }
```