UPSTREAM: wireguard: selftests: test multiple parallel streams

In order to test ndo_start_xmit being called in parallel, explicitly add
separate tests, which should all run on different cores. This should
help tease out bugs associated with queueing up packets from different
cores in parallel. Currently, it hasn't found those types of bugs, but
given future planned work, this is a useful regression to avoid.

Bug: 254441685
Fixes: e7096c13 ("net: WireGuard secure network tunnel")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
(cherry picked from commit d5a49aa6)
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: Iba3be1074fc44f47eb06d5776919318acf548511