grub2: re-add test module

grub2: re-add test module

It seems more hardware needs early load of firmware when initialised
to work properly (at least Intel hardware). One of previous case is CPU
microcode, which this series[1] tried to change. The second one is Intel
graphics IC, which needs firmware for controlling DMC circuit (switch
conncted display to DC6 power state). As it stands, the i915 module is
built-in and it seems the hardware can't cope with firmware loaded
later from rootfs, it needs to be supplied when the module is loaded.
Unfortunately we need bootloader to handle the load of firmware in this
case, but as previously mentioned series[1], there was an error when
initrd was hardcoded, instead of testing existence for it and then
loading. To remedy this in later the 55b808e0c4be ('x86: image: add test
module to bootloader') was commited. Which was later accidentally
dropped when grub2 image creation was moved to packages. Therefore bring
back test module, so we can test for cases of existing firmware in
grub.cfg.

1. https://patchwork.ozlabs.org/project/openwrt/cover/20181120162044.16371-1-tomek_n@o2.pl

Fixes: 5a5df62d95f5 ("x86/grub2: move grub2 image creation to package")
Signed-off-by: Tomasz Maciej Nowak <tmn505@gmail.com>