sandbox: Add basic test for command execution

Since run_command() and run_command_list() are important and a little
confusing, add some basic tests to check that the behaviour is correct.

Note: I am not sure that this should be committed, nor where it should go
in the source tree. Comments welcome.

To run the unit tests use the ut_cmd command available in sandbox:

make sandbox_config
make
./u-boot -c ut_cmd

(To test both hush and built-in parsers, you need to manually change
CONFIG_SYS_HUSH_PARSER in include/configs/sandbox.h and build/run again)

Signed-off-by: Simon Glass <sjg@chromium.org>