net/netip: add test for AddrFromSlice

net/netip: add test for AddrFromSlice

AddrFromSlice is not covered by any other test so far.

Change-Id: I91034c6cac95a023fc419c855873a395b1afdab7
Reviewed-on: https://go-review.googlesource.com/c/go/+/435916
Reviewed-by: Carlos Amedee <carlos@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>