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_test.go

@@ -1,39 +0,0 @@
-package ndt7
-
-import (
-	"io"
-	"time"
-
-	"github.com/gorilla/websocket"
-)
-
-type mockableConnMock struct {
-	NextReaderMsgType       int
-	NextReaderErr           error
-	NextReaderReader        func() io.Reader
-	ReadDeadlineErr         error
-	WriteDeadlineErr        error
-	WritePreparedMessageErr error
-}
-
-func (c *mockableConnMock) NextReader() (int, io.Reader, error) {
-	var reader io.Reader
-	if c.NextReaderReader != nil {
-		reader = c.NextReaderReader()
-	}
-	return c.NextReaderMsgType, reader, c.NextReaderErr
-}
-
-func (c *mockableConnMock) SetReadDeadline(time.Time) error {
-	return c.ReadDeadlineErr
-}
-
-func (c *mockableConnMock) SetReadLimit(int64) {}
-
-func (c *mockableConnMock) SetWriteDeadline(time.Time) error {
-	return c.WriteDeadlineErr
-}
-
-func (c *mockableConnMock) WritePreparedMessage(*websocket.PreparedMessage) error {
-	return c.WritePreparedMessageErr
-}