net/http/httptest: httptest on Windows is IPv4-specific

What version of Go are you using (go version)?

go version go1.11.1 windows/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

windows/amd64

What did you do?

Run a test leveraging net/http/httptest on a host with IPv6 but without IPv4.

To reproduce:

1. As administrator, run netsh interface ipv4 uninstall
2. Reboot
3. Execute an httptest as in https://play.golang.org/p/xlMUEEhgj5s

What did you expect to see?

Test would pass, since the client and server code were both valid, and pass when IPv4 is available.

What did you see instead?

2019/01/15 17:26:59 Get http://[::1]:49679: dial tcp [::1]:49679: failed to find ConnectEx: An address incompatible with the requested protocol was used.