Add HttpClient test with IPv6 Uris

[stephentoub]

Adds support to the loopback server for binding to IPv6 local addresses, then adds a test that uses both ::1 and a link-local address for the current machine.

cc: @davidsh, @ericeil, @mellinoe, @mikeharder
(Relies on #8462; won't pass on Unix until that's merged.)

[davidsh]

nit: Our coding guidelines currently don't permit these single line if/else statements but rather require braces, etc.

[stephentoub]

Ok, I'll expand it.

[davidsh]

LGTM

[stephentoub]

Test Innerloop OSX Debug Build and Test please (build machine went offline)
Test Innerloop OSX Release Build and Test please (build machine went offline)

[stephentoub]

@mmitche, do our Ubuntu VMs have IPv6 enabled? These tests pass for me locally, but when trying to bind to an IPv6 address, it's failing in the lab:

System.Net.Http.Functional.Tests.HttpClientHandlerTest.GetAsync_IPv6Uri_Success(address: fe80::20d:3aff:fe32:2f5d) [FAIL]
        System.Net.Sockets.SocketException : Invalid argument
        Stack Trace:
              at System.Net.Sockets.Socket.DoBind(EndPoint endPointSnapshot, SocketAddress socketAddress)
              at System.Net.Sockets.Socket.Bind(EndPoint localEP)
              at System.Net.Http.Functional.Tests.LoopbackServer.CreateServerAsync(Func`3 funcAsync, IPEndPoint& localEndPoint, Options options)

[davidsh]

do our Ubuntu VMs have IPv6 enabled? These tests pass for me locally, but when trying to bind to an IPv6 address, it's failing in the lab

This brings up an interesting issue. In the past, we (networking team) had to deal with IPv4 and/or IPv6 being disabled/un-installed when doing networking things in .NET Framework. The current version of Windows, for example, can probably still disable (not un-install) IPv6 protocol bindings. Sounds like you can do similar in *Nix.

So, we should make sure our code is resilient to this and add tests for it. I suggest "unit" tests and not functional tests because then you can easily simulate a machine with only IPv4 and no IPv6 and v.v.

[davidsh]

cc: @CIPop

[mmitche]

How would I find out?

[mmitche]

@dotnet-bot test this please

[stephentoub]

Oh, nevermind, it is enabled. The problem is that the fix I made yesterday to ensure that the scope ID is included in an IPAddress isn't yet in the package for System.Net.NetworkInformation being used by this test suite. Once we move to the newer packages, this should be fixed.