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 <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Reviewed-by: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go