pr: Check the correct timestamp in test_with_pr_core_utils_tests

[BenWiederhake]

The test test_pr::test_with_pr_core_utils_tests is flaky, and causes real problems.

Here's a branch that makes the bug 100% reproducible, by injecting "lag" at critical moments: https://github.com/BenWiederhake/coreutils-rs/tree/historic/demo-pr-test-race

The bug is best explained by the fix:

     let input_file_path = input_file.first().unwrap();
     let test_file_path = expected_file.first().unwrap();
-    let value = file_last_modified_time(&scenario, test_file_path);
+    let value = file_last_modified_time(&scenario, input_file_path);
     let mut arguments: Vec<&str> = …;
     arguments.extend(input_file.clone());
     // "value" is then used to construct the expected output.

In other words: value is the last modified time of the wrong file.

All other tests in test_pr seem to use the correct file, so this should fix this issue for good.

[tertsdiepraam]

Whoa, great job finding the issue here! Seeing the CI green is so nice!

[sylvestre]

+1!