Fix waiting for the server to start in t/author/http-server.t

Use Net::EmptyPort directly, since we don't need any of the actual
testing stuff from Test::TCP. This also lets us control the sleep
interval and retries, so we don't have to loop ourselves as well.

Net::EmptyPort started exporting wait_port in 1.27, but doesn't have a
$VERSION of its own, so declare the author dependency on Net::TCP 1.27.