tests: torture tests don't compile on 32-bit architectures

[decathorpe]

This error occurs when trying to compile / run the test suite on i686 or armv7hfp / Linux:

error[E0308]: mismatched types
  --> tests/torture.rs:17:51
   |
17 |     let mut rng = rand::rngs::SmallRng::from_seed(seed);
   |                                                   ^^^^ expected an array with a fixed size of 16 elements, found one with 32 elements
For more information about this error, try `rustc --explain E0308`.

Note that this error is from compiling num-bigint-dig 0.7.0, but the affected code has not changed since then.

It looks like rand::rngs::SmallRng::from_seed expects a 16-element array on 32-bit architectures and a 32-element array on 64-bit architectures.