[U-Boot] Sandbox dm tests leave sandbox in a bad state

Hi Simon,

I tracked a segfault down to the entire DM being left torn down after the last test. This means anything that wants to use the DM is hosed. I'm seeing a segfault in the sandbox related to a driver name being NULL when running "dm tree".

I also noticed that the USB test dm_test_usb_flash can't run more than once without crashing.

Thoughts? Are these just expected and accepted limitations?

Thanks, -Joe