dm: test: Don't fail when tests are skipped due to build

At present tests that are marked as only for livetree fail when executed
on sandbox_flattree. They cannot actually be executed, but we should not
resport them as 'not found', since this causes errors. Instead, they
should be silently skipped.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>