unix: fix out of bounds memory access in tests

sockaddrIUCVToAny used the wrong size to for copying to RawSockaddrAny.
We found this by running the test with ASAN.

Change-Id: I2a2ec28f9bd55aeba4ddcb4243ad61ec010087df
Reviewed-on: https://go-review.googlesource.com/c/sys/+/724280
Reviewed-by: Michael Stapelberg <stapelberg@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>