dm: test: Update Makefile conditions

At present most of the tests in test/Makefile are dependent on
CONFIG_SANDBOX. But this is not ideal since they rely on commands being
available and SPL does not support commands.

Use CONFIG_COMMAND instead. This has the dual purpose of allowing these
tests to be used on other boards and allowing SPL to skip them.

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