test/dm: clear unit test failure count each run

test/dm: clear unit test failure count each run

The ut command prints a test failure count each time it is executed.
This is stored in a global variable which is never reset. Consequently,
the printed failure count accumulates across runs. Fix this by clearing
the counter each time "ut" is invoked.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>