Skip if_wg regression test if module doesn't exist

[olivier]

Summary

When building light FreeBSD without if_wg this regression test fail because it can't load the module.

Test Plan

1. mv /boot/kernel/if_wg.ko /boot/kernel/if_wg.ko.bak
2. kyua test sys/net/if_wg

sys/net/if_wg:wg_basic -> skipped: This test requires if_wg and could not load it [0.008s]
sys/net/if_wg:wg_key_peerdev_makeshared -> skipped: This test requires if_wg and could not load it [0.008s]
sys/net/if_wg:wg_key_peerdev_shared -> skipped: This test requires if_wg and could not load it [0.008s]
Results file id is usr_tests.20230206-185148-152438
Results saved to /root/.kyua/store/results.usr_tests.20230206-185148-152438.db
3/3 passed (0 failed)