There's no reason to use a bootloader when testing kexec, this is a feature that reboots *directly* in the kernel, if anything, we should just direct boot the kernel and reboots in the kernel. A bootloader test really makes sense to test "default" systemctl kexec behavior which is already broken because systemctl kexec will read the ESP to determine what to kexec by default.
*** NixOS *** NixOS is a Linux distribution based on the purely functional package management system Nix. More information can be found at https://nixos.org/nixos and in the manual in doc/manual.