tests: fix sorting kernel version
tests: fix sorting kernel version

Debian now has 4.9 and 4.19 kernels installed, so `sort -n` sorts them
wrong.

QubesOS/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin