test: cmd: fdt: Generate fuller DT internally and switch fdt get value to it

Implement function to generate internal test DT fragment and switch
the 'fdt get value' test to this instead of depending on the sandbox
DT. Rename clk-test node to test-node node. This FDT fragment will be
reused by other tests. No functional change.

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