nixos/wireguard: test against multiple kernel versions

When testing WireGuard updates, I usually run the VM-tests with
different kernels to make sure we're not introducing accidental
regressions for e.g. older kernels.

I figured that we should automate this process to ensure continuously
that WireGuard works fine on several kernels.

For now I decided to test the latest LTS version (5.4) and
the latest kernel (currently 5.6). We can add more kernels in the
future, however this seems to significantly slow down evaluation and
time.

The list can be customized by running a command like this:

   nix-build nixos/tests/wireguard --arg kernelVersionsToTest '["4.19"]'

The `kernelPackages` argument in the tests is null by default to make
sure that it's still possible to invoke the test-files directly. In that
case the default kernel of NixOS (currently 5.4) is used.