test: rng: Add a UT testcase for the rng command

The 'rng' command dumps a number of random bytes on the console. Add a
set of tests for the 'rng' command. The test function performs basic
sanity testing of the command.

Since a unit test is being added for the command, enable it by default
in the sandbox platforms.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index a8260aa..3625ff2 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1964,6 +1964,7 @@
 config CMD_RNG
 	bool "rng command"
 	depends on DM_RNG
+	default y if SANDBOX
 	select HEXDUMP
 	help
 	  Print bytes from the hardware random number generator.