Extend support for randomness generation in tests

Catch2 currently has a basic support for generating reproducible random
numbers for tests. Right now, Catch2 provides only uniform distributions
over integers and real numbers projected into floating point.

This could be greatly extended with different distributions, different
type of generators (e.g. generating uniformly from floats, instead of
real numbers projected to floats) and adding some building blocks for
users to build their own random generators.