Allow the test suite to customise its random seed

Allow the test suite to customise its random seed

This will only affect the property based tests, not the test vectors
themselves.  The idea is to let paranoid users run the test suite with
lots and lots of different streams of random numbers, just to be safe.

Test vector generation could undergo a similar transformation, though it
is less likely to be worth the trouble (we'd have to generate the test
vectors, compile the test suite all over again).