test: cmd: fdt: Test fdt get name

Add 'fdt get name' test which works as follows:
- Create fuller FDT, map it to sysmem
- Get name of / node 0, 1 and /clk-test node 0
- Compare output and validate the node name
- Get name of / node 2 and /clk-test node 1
- Compare output and validate the node is not present
- Get name of / node -1 and /clk-test node -1
- Compare output and validate the node name equals node 0 name
- Check nonexistent node, verify the command errors out

The test case can be triggered using:
"
./u-boot -Dc 'ut fdt'
"
To dump the full output from commands used during test, add '-v' flag.

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