efi_selftest: multi part device path to text

Test EFI_DEVICE_PATH_TO_TEXT_PROTOCOL.ConvertDevicePathToText() for a multi
part device path.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>