Check that cube.copy() makes a fresh mask.

Additional to https://github.com/SciTools/iris/pull/1411
Addresses comment : https://github.com/SciTools/iris/pull/1411/files#r19330126

You might also be able to use ```self.assertIsNot(cube.data.mask, cube_copy.data.mask)```, but go with whatever you feel is clearer.