wireguard: selftests: increase default dmesg log size

The selftests currently parse the kernel log at the end to track
potential memory leaks. With these tests now reading off the end of the
buffer, due to recent optimizations, some creation messages were lost,
making the tests think that there was a free without an alloc. Fix this
by increasing the kernel log size.

Fixes: 24b70eee ("wireguard: use synchronize_net rather than synchronize_rcu")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>