wireguard: selftests: support UML

This shoud open up various possibilities like time travel execution, and
is also just another platform to help shake out bugs.

Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>