fix indeterminacy in FiniteSet

This fixes #7456. I downloaded 3.4 and found a failing pythonhashseed for test_rv. After applying these mods the test no longer occurs so I think it's taken care of now. I reverted the sorting of random_symbols as it was not needed to fix this issue.

In addition, I noticed that a deprecation warning from numpy is emitted during testing because non-integer dimensions were being used to create a matrix, so I used randint instead.