Fix blessing of test output in subdirectories

The core issue was the usage of `reference_file_path.file_name()`, which
provided a non-existent path if the file to be updated was in a
subdirectory.

Instead we have to provide the whole path after 'tests/ui/' as the
'filename'. This part of the path is called `test_name` in the code now.

clippy_dev/src/bless.rs

@@ -28,15 +28,17 @@ pub fn bless() {
             .filter_map(Result::ok)
             .filter(|f| f.path().extension() == Some(OsStr::new("rs")))
             .for_each(|f| {
-                update_reference_file(f.path().with_extension("stdout"));
-                update_reference_file(f.path().with_extension("stderr"));
-                update_reference_file(f.path().with_extension("fixed"));
+                let test_name = f.path().strip_prefix(test_dir).unwrap();
+
+                update_reference_file(f.path().with_extension("stdout"), test_name.with_extension("stdout"));
+                update_reference_file(f.path().with_extension("stderr"), test_name.with_extension("stderr"));
+                update_reference_file(f.path().with_extension("fixed"), test_name.with_extension("fixed"));
             });
     }
 }
 
-fn update_reference_file(reference_file_path: PathBuf) {
-    let test_output_path = build_dir().join(PathBuf::from(reference_file_path.file_name().unwrap()));
+fn update_reference_file(reference_file_path: PathBuf, test_name: PathBuf) {
+    let test_output_path = build_dir().join(test_name);
     let relative_reference_file_path = reference_file_path.strip_prefix(clippy_project_root()).unwrap();
 
     // If compiletest did not write any changes during the test run,