Secure erase tests

Implement simple secure erase tests. Perform IO that will trigger
copying of Data buffers and make sure OCF calls secure erase on them.

Signed-off-by: Jan Musial <jan.musial@intel.com>

tests/functional/pyocf/utils.py

@@ -62,6 +62,9 @@ class Size:
     def __int__(self):
         return self.bytes
 
+    def __index__(self):
+        return self.bytes
+
     @classmethod
     def from_B(cls, value):
         return cls(value)