When a positive integer limit is given, `rand` method of a RNG object
is expected to return a value between 0 and the limit (exclusive).

Fix shuffle_spec.rb like as the similar code in sample_spec.rb, and
add tests for greater values.

TODO:
- Return a value that is equal to or greater than the limit given to
  the RNG object.
- Extract common code about RNG objects to a shared file.