fdt: Add tests for fdtdec

The fdtdec_find_aliases_for_id() function is complicated enough that
it really should have some tests. This does not necessarily need to be
committed to U-Boot, but it might be useful.

(note there are a few minor inconsistencies with this patch which will be
cleaned up when the USB series is applied)

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>