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/download.go

@@ -11,7 +11,7 @@ import (
 )
 
 type downloadManager struct {
-	conn            mockableConn
+	conn            wsConn
 	maxMessageSize  int64
 	maxRuntime      time.Duration
 	measureInterval time.Duration
@@ -20,7 +20,7 @@ type downloadManager struct {
 }
 
 func newDownloadManager(
-	conn mockableConn, onPerformance callbackPerformance,
+	conn wsConn, onPerformance callbackPerformance,
 	onJSON callbackJSON,
 ) downloadManager {
 	return downloadManager{