Turning off IPv6 causes the tests to fail because the Go
`net.Listen()` function obtains a `[::]` address when using
a `localhost:0` bind address. Looking through the Go source
code, I don't see a good way to fix that.
This change adds a test flag to allow users to specify the
network name when running the test:
```sh
go test github.com/pilosa/pilosa/server -network tcp4
```
