Add unit tests for hash sample optimization

* Instead of keeping md5 samples separate, merge them as one hash computed from the various selected chunks we picked.
* We don't need to keep a boolean to see whether or not the user chose to optimize; we can simply compare the value of the threshold, since 0 means no optimization currently active.

core/tests/base.py

@@ -88,6 +88,7 @@ class NamedObject:
         self.size = size
         self.md5partial = name
         self.md5 = name
+        self.md5samples = name
         if with_words:
             self.words = getwords(name)
         self.is_ref = False