wireguard: selftests: load nf_conntrack if not present

wireguard: selftests: load nf_conntrack if not present

Some distros may not load nf_conntrack by default, which will cause
subsequent nf_conntrack sets to fail. Load this module if it is not
already loaded.

Fixes: e7096c131e51 ("net: WireGuard secure network tunnel")
Signed-off-by: Hangbin Liu <[email protected]>
Reviewed-by: Simon Horman <[email protected]>
[ Jason: add [[ -e ... ]] check so this works in the qemu harness. ]
Signed-off-by: Jason A. Donenfeld <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>