Auto merge of #9475 - PaulDance:fix-test-support-warning, r=ehuss

Fix Url::into_string deprecation warning

```rust
warning: use of deprecated associated function `url::Url::into_string`: use Into<String>
   --> src/registry.rs:183:26
    |
183 |                 dl_url().into_string(),
    |                          ^^^^^^^^^^^
    |
    = note: `#[warn(deprecated)]` on by default
```

is being emitted when running `cargo build` directly from the `crates/cargo-test-support` or the `crates/mdman` crate. This simply proposes a switch to the recommended method in order to resolve the deprecation warning.