test: dm: Don't bail on all tests if one test fails

There's not much point in having a failure count if we always give up on
the first failure. Also stop clearing the entire state between tests.

Make sure that any failures are still passed out to the command line.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Simon Glass <sjg@chromium.org>