wireguard: selftests: set fake real time in init

Not all platforms have an RTC, and rather than trying to force one into
each, it's much easier to just set a fixed time. This is necessary
because WireGuard's latest handshakes parameter is returned in wallclock
time, and if the system time isn't set, and the system is really fast,
then this returns 0, which trips the test.

Turning this on requires setting CONFIG_COMPAT_32BIT_TIME=y, as musl
doesn't support settimeofday without it.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>