chore: Updated source to follow clippy suggestions.

tests/integration_tests.rs

@@ -131,7 +131,7 @@ fn all_exercises_require_confirmation() {
             file.read_to_string(&mut s).unwrap();
             s
         };
-        source.matches("// I AM NOT DONE").next().expect(&format!(
+        source.matches("// I AM NOT DONE").next().unwrap_or_else(|| panic!(
             "There should be an `I AM NOT DONE` annotation in {:?}",
             path
         ));