Add CTS tests for Hardware RNG (/dev/hw_random).

* The tests assert that the HW RNG is configured correctly, if
  present.
* The tests do not check the quality of its output, because apps
  cannot access its output and the system components that can (init
  and EntropyMixer) do not rely on the HW RNG's quality.

Change-Id: Idf1a6678081300ccd91ee2175bf540d6a35901c1