Adds missing test coverage for rustdoc's `--test-builder` option.
The existing test only covered the error case (non-executable builder). This PR
adds:
- A custom test builder that logs arguments and forwards to rustc
- A test verifying that `--test-builder` successfully invokes the custom
builder with rustc-style arguments
- Improved comments explaining both the error and success test scenarios
The test validates that custom builders can properly intercept and handle
doctest compilation.
Signed-off-by: Osama Abdelkader <osama.abdelkader@gmail.com>
