Fedora 34 and thus RHEL 9 have adopted the unified grub config
scheme[1], where the main config is always placed in the same
location across all platforms, i.e. `boot/grub2`, and a stub
config that redirects to the main config is placed into the ESP.
osbuild has always done that in the case of hybrid boot, but not
for pure EFI systems. The new `uefi.unified` config option can
be used to select that new unified scheme even for the case of
pure EFI systems (aarch64 or non-hybrid boot).
Add a simple test for the grub stage.
[1] https://fedoraproject.org/wiki/Changes/UnifyGrubConfig
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