x86: image: add test module to bootloader

x86: image: add test module to bootloader

It was previously added in 546fced, which was part of "intel-microcode:
load as early as possible" series.
Unfortunately the conditionals added to GRUB config caused error on boot,
because on sysupgrade, bootloader is not updated and is left with old
features/modules. Since this module is needed for early microcode load
and transition to this needs to be done step by step, enable the test
module now, so that every newly created image has it already embedded.

Signed-off-by: Tomasz Maciej Nowak <tomek_n@o2.pl>