refactor(ndt7): use netxlite rather than netx (#768)

This diff required us to move some code around, but no major
change actually happened, except better tests.

While there, I also slightly refactored ndt7's implementation and
removed the ProxyURL setting, which was actually unused.

See https://github.com/ooni/probe/issues/2121

internal/engine/experiment/ndt7/mockable.go

@@ -1,16 +0,0 @@
-package ndt7
-
-import (
-	"io"
-	"time"
-
-	"github.com/gorilla/websocket"
-)
-
-type mockableConn interface {
-	NextReader() (int, io.Reader, error)
-	SetReadDeadline(time.Time) error
-	SetReadLimit(int64)
-	SetWriteDeadline(time.Time) error
-	WritePreparedMessage(*websocket.PreparedMessage) error
-}