Fixing random genRandomTest method in TestBerlekampWelchDecode

...y points can be less than wanted. Fixed this by generating random and distinct integers with a helper method.

Note: code line 47 is a little bit faulty, since there exists no fair distribution (i.e. throwing a 256-sided dice and doing modulo isn't well distributed) . We can do better, but IMO it shouldn't be a problem in this test.