[C++] Unify the sets of random generators in testing/random.h

### Describe the enhancement requested

Unify random data generators while considering their distinct use-cases: testing+fuzzying and benchmarks.

Benchmarking data should be more similar to what's more common in practice so the benchmarks reflect reality.

Testing and fuzzying data should cover more corner cases which can sometimes be in conflict with the goals of benchmarking.

### Component(s)

C++