test: cmd: fdt: Test alias resolution in 'fdt get value'

The 'fdt' command help contains the following note:
"
Dereference aliases by omitting the leading '/', e.g. fdt print ethernet0.
"
Add test for it.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Simon Glass <sjg@chromium.org>