test: rng: Add basic test for random number generator(rng) uclass

Add a unit test for testing the rng uclass functionality using the
sandbox rng driver.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>