Fix minor issue with cleaning up directories.

One of the previously made changes to cleaning up directories was that
empty directories were deleted. This was necessary, because otherwise
there would be a growing number of directories as files get deleted
after reaching an age of seven weeks.

However, this change should not have included deleting the cleaned up
directory itself. In practice, this will not happen. But in tests it's
certainly possible that a directory is empty and then gets deleted.
This leads to all sorts of problems in tests.

The fix is to limit deleting empty directories to subdirectories.
That's what this commit does.

src/main/java/org/torproject/metrics/collector/persist/PersistenceUtils.java

@@ -132,7 +132,8 @@ public class PersistenceUtils {
       @Override
       public FileVisitResult postVisitDirectory(Path dir, IOException exc)
           throws IOException {
-        if (!Files.list(dir).findFirst().isPresent()) {
+        if (!pathToClean.equals(dir)
+            && !Files.list(dir).findFirst().isPresent()) {
           Files.delete(dir);
         }
         return FileVisitResult.CONTINUE;