tests: Add efi_loader hello world test

Now that we have working network tests and a hello world efi application
built inside our tree, we can automatically test that efi binary running
inside of U-Boot.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>