net/netip: use IPv4Unspecified in TestIPProperties

net/netip: use IPv4Unspecified in TestIPProperties

Also inline the single-use unspecified{4,6} variables.

Change-Id: I5992273031e0b8db1bc6f5de8fce669310226ee9
Reviewed-on: https://go-review.googlesource.com/c/go/+/435915
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>