test: avoid kernel-install add-all

test: avoid kernel-install add-all

The man page of `kernel-install` says about `add-all`: "This is the same
as add, but invokes the operation iteratively for every installed kernel
in /usr/lib/modules/. This operation is only supported on systems where
the kernel image is installed in
/usr/lib/modules/KERNEL-VERSION/vmlinuz."

Debian and Ubuntu do not ship the kernel images in `/usr/lib/modules/`
and therefore `add-all` is not supported there.

Since the test cases already determine the kernel version, search for
the kernel image and use `kernel-install add` instead of `add-all`.