cleanup: mark more integration tests as !short mode (#755)

The objective is to make PR checks run much faster.

See https://github.com/ooni/probe/issues/2113 for context.

Regarding netxlite's tests:

Checking for every commit on master or on a release branch is
good enough and makes pull requests faster than one minute since
netxlite for windows is now 1m slower than coverage.

We're losing some coverage but coverage from integration tests
is not so good anyway, so I'm not super sad about this loss.

internal/engine/experiment/ndt7/download_test.go

@@ -117,6 +117,9 @@ func (r *invalidJSONReader) Read(p []byte) (int, error) {
 }
 
 func TestDownloadOnJSONLoop(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skip test in short mode")
+	}
 	mgr := newDownloadManager(
 		&mockableConnMock{
 			NextReaderMsgType: websocket.TextMessage,

internal/engine/experiment/ndt7/ndt7_test.go

@@ -187,6 +187,9 @@ func TestFailUpload(t *testing.T) {
 }
 
 func TestDownloadJSONUnmarshalFail(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skip test in short mode")
+	}
 	measurer := NewExperimentMeasurer(Config{noUpload: true}).(*Measurer)
 	var seenError bool
 	expected := errors.New("expected error")

internal/engine/experiment/ndt7/upload_test.go

@@ -73,6 +73,9 @@ func TestUploadWritePreparedMessageSubsequentFailure(t *testing.T) {
 }
 
 func TestUploadLoop(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skip test in short mode")
+	}
 	mgr := newUploadManager(
 		&mockableConnMock{},
 		defaultCallbackPerformance,