Add assert to RandomContentFile, ensure size >= digest_size

It clearly will not work to have the size smaller than the digest
size, so we shouldn't allow it.

s3tests/realistic.py

@@ -17,6 +17,11 @@ class RandomContentFile(object):
         self.digest = None
         self.last_read = 0
 
+        assert self.size >= self.digest_size, "Can not create RandomContentFile: size ({size}) must be >= digest_size ({digest_size})".format(
+            size=self.size,
+            digest_size=self.digest_size
+            )
+
     def seek(self, offset):
         assert offset == 0
         self.random.seed(self.seed)