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>
diff --git a/Makefile b/Makefile
index eb37ea1..20fa272 100644
--- a/Makefile
+++ b/Makefile
@@ -304,6 +304,7 @@
 LIBS += lib/libfdt/libfdt.o
 LIBS += api/libapi.o
 LIBS += post/libpost.o
+LIBS += test/libtest.o
 
 ifneq ($(CONFIG_AM33XX)$(CONFIG_OMAP34XX)$(CONFIG_OMAP44XX)$(CONFIG_OMAP54XX),)
 LIBS += $(CPUDIR)/omap-common/libomap-common.o