When you test a new kernel or a new OS, it is important to make sure that your computer can boot even if the new system is unbootable. This is crucial especially if you maintain servers or remote systems. To accomplish this goal, you need to set up two things:
The former requirement is very specific to each OS, so this documentation does not cover that topic. It is better to consult some backup tools.
So let's see the GRUB part. There are two possibilities: one of them is quite simple but not very robust, and the other is a bit complex to set up but probably the best solution to make sure that your system can start as long as GRUB itself is bootable.