test: cmd: fdt: Test fdt chosen

test: cmd: fdt: Test fdt chosen

Add 'fdt chosen' test which works as follows:
- Create basic DT, map it to sysmem
- Print /chosen node, verify it is nonexistent
- Create chosen node
- Print /chosen node, verify it contains only version
- Create /chosen node with initrd entries
- Print /chosen node, verify it contains version and initrd entries

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>