net/netip: cover more AddrPort.String cases in tests

net/netip: cover more AddrPort.String cases in tests

TestInvalidAddrPortString currently only tests for invalid AddrPorts.
Add some valid cases as well to improve test coverage.

Change-Id: Iaa9192e48a61daed6f7ce7d680d602a021570bdc
Reviewed-on: https://go-review.googlesource.com/c/go/+/613795
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>