test rand a bit more

[RalfJung]

Currently fails, blocked on rust-random/rand#779

[RalfJung]

This is now blocked on a new release of rand containing rust-random/rand#780 and rust-random/rand#784. @dhardy what are your plans there?

[dhardy]

@RalfJung the next release from the master branch will be version 0.7, which is tracked by rust-random/rand#715. I hope it won't take us much longer to get there, but can't make any promises.

The alternative would be to back-port these changes to 0.6.

[RalfJung]

I updated this to the 0.7 pre-release, so I hope this should work now. :)