Move a resaved test file's mtime ahead, so the scan cannot miss it

`a_resaved_file_owes_a_reread_and_loses_its_stale_hash` failed once in a
full dr-catalog run (updated 0, expected 1) and passed three times alone.
The incremental scan tells a changed file by its mtime at whole-second
resolution, and the `resave` helper wrote and renamed the file within
the same second as the scan before it, so on a fast enough pass the
resave looked like no change at all.

The helper now sets the file's and its folder's mtime two seconds ahead,
which is what a real resave some time after a scan looks like. Only the
test helper changes; the scan's rule is right for real files.

core/dr-catalog/src/walk.rs

@@ -798,6 +798,17 @@ mod tests {
             let tmp = target.with_extension("tmp");
             fs::write(&tmp, bytes).expect("write");
             fs::rename(&tmp, &target).expect("rename");
+            // A scan tells a changed file by its mtime, at whole-second
+            // resolution; a resave landing in the same second as the scan
+            // before it looks unchanged, and the test fails when the machine
+            // is fast enough. Two seconds ahead, on the file and its folder,
+            // is what a real resave some time later would look like.
+            let later = std::time::SystemTime::now() + std::time::Duration::from_secs(2);
+            for p in [target.as_path(), target.parent().expect("parent")] {
+                fs::File::open(p)
+                    .and_then(|f| f.set_modified(later))
+                    .expect("set mtime");
+            }
             self
         }