test: rewrite doc test

Line numbering has changed in Rust 1.71 in the doc tests, due to an
extra inserted `#[allow(unused_extern_crates)]` line.

Don't test for the exact line number anymore.

Signed-off-by: Harald Hoyer <harald@hoyer.xyz>

src/lib.rs

@@ -60,17 +60,16 @@
 //! fn main() {
 //!     if let Err(e) = process_config_file() {
 //!         eprintln!("Error:\n{:?}", e);
-//! #       assert_eq!(
+//! #       let s = format!("{:?}", e);
-//! #           format!("{:?}\n", e),
+//! #       let lines = s.lines().collect::<Vec<_>>();
-//! #           "\
+//! #       assert_eq!(lines.len(), 5);
-//! # src/lib.rs:16:51: read the config file\n\
+//! #       assert!(lines[0].starts_with("src/lib.rs:"));
-//! # Caused by:\n\
+//! #       assert_eq!(lines[1], "Caused by:");
-//! # src/lib.rs:9:47: Reading file: \"foo.txt\"\n\
+//! #       assert!(lines[2].starts_with("src/lib.rs:"));
-//! # Caused by:\n\
+//! #       assert_eq!(lines[3], "Caused by:");
-//! # Os { code: 2, kind: NotFound, message: \"No such file or directory\" }\n\
+//! #       assert_eq!(lines[4], "Os { code: 2, kind: NotFound, message: \"No such file or directory\" }");
-//! #            ",
-//! #        );
 //!     }
+//! #   else { panic!(); }
 //! }
 //! ```
 //!