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 <liuhangbin@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
[ Jason: add [[ -e ... ]] check so this works in the qemu harness. ]
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Link: https://patch.msgid.link/20241117212030.629159-4-Jason@zx2c4.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>