net/netip: cover IPv4Unspecified in TestAddrWellKnown

Like IPv6Unspecified which is already covered in that test.

Change-Id: I2bac4c50577c4c2c91cea26b9fbed88327cef516
Reviewed-on: https://go-review.googlesource.com/c/go/+/589595
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>
Auto-Submit: Tobias Klauser <[email protected]>

Author: tklauser

src/net/netip/netip_test.go

@@ -788,6 +788,11 @@ func TestAddrWellKnown(t *testing.T) {
 		ip   Addr
 		std  net.IP
 	}{
+		{
+			name: "IPv4 unspecified",
+			ip:   IPv4Unspecified(),
+			std:  net.IPv4zero,
+		},
 		{
 			name: "IPv6 link-local all nodes",
 			ip:   IPv6LinkLocalAllNodes(),