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/wsconn.go

@@ -0,0 +1,17 @@
+package ndt7
+
+import (
+	"io"
+	"time"
+
+	"github.com/gorilla/websocket"
+)
+
+// weConn is the interface of gorilla/websocket.Conn
+type wsConn interface {
+	NextReader() (int, io.Reader, error)
+	SetReadDeadline(time.Time) error
+	SetReadLimit(int64)
+	SetWriteDeadline(time.Time) error
+	WritePreparedMessage(*websocket.PreparedMessage) error
+}