Invoke each "ut"-based unit test as a separate pytest.

Now that the DM unit test runs under test/py, remove the manual shell
script that invokes it.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org> # v2, on sandbox