Do not use std::default_random_engine

[ptheywood]

std::default_random_engine is implementation defined, so different sequences are used on different platforms which is not ideal.

Most occurences were updated to use std::mt19937_64, the 64 bit mersenne twister implementation in the standard library, but there are currenlty 39 occurances in the test suite of the default being used (likely in PR's or based on tests from before the change to 64 bit mt was made).

These should be updated.