test/py: Fix efidebug related tests

commit cbea241e("efidebug: add multiple device path instances on Boot####")
slightly tweaked the efidebug syntax adding -b, -i and -s for the boot
image, initrd and optional data.
The pytests using this command were adapted as well. However I completely
missed the last "" argument, which at the time indicated the optional data
and needed conversion as well.  This patch is adding the missing -s flag
and the tests are back to normal.

Fixes: cbea241e("efidebug: add multiple device path instances on Boot####")
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviwed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>