change endpoint config into setup block style

[slapers]

Hi,

I hope you don't mind me just opening a PR without any discussions on this. But this PR gives another option of handling the @endpoint config.

I think this PR is very close to the #231 but I decided to put the phoenix_endpoint in the conn private assigns, this way It can be configured in the setup block.

In the PR I removed the config entirely, but its trivial to add it again to keep it backwards compatible.

 setup tags do
    MyApp.DataCase.setup_sandbox(tags)
    {:ok, conn: Phoenix.ConnTest.build_conn() |> PhoenixTest.put_endpoint(MyAppWeb.Endpoint)}
 end