sandbox: Avoid binman error when run without device tree

With sandbox, U-Boot can be run without a device tree (i.e. no -d or -T
parameter). In this case an empty device tree is created for convenience.
With a recent change this causes an error due to the missing '/binman'
node.

Add this node to avoid the problem, as well as a test that U-Boot can
be run without a device tree.

Fixes: 059df562 ("arch: Kconfig: imply BINMAN for SANDBOX")
Fixes: https://source.denx.de/u-boot/u-boot/-/issues/11


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