binman: ftest: Add test for u_boot_spl_pubkey_dtb

Add test for u_boot_spl_pubkey_dtb. The test adds a public key to the
dtb and checks if the required nodes will be added to the images dtb.

Signed-off-by: Lukas Funke <lukas.funke@weidmueller.com>
Reviewed-by: Simon Glass <sjg@chromium.org>