tests: random: Move and rename rng test

Random number generator is a subsystem and although it is mostly used
in crypto, this is not its only utility.

Move the current random number generator test to it is own space
(tests/subsys/random) and rename it to rng.

We need more and better tests for rng, this is an initial commit
to organize it and get ready for further tests.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>