Missing check for "OPENCV_ENABLE_NONFREE" in "TestSIFT()"

[pericles-tpt]

Description

There seems to be a missing check for "OPENCV_ENABLE_NONFREE" in TestSIFT(), which according to the OpenCV documentation is a required option to use the SIFT algorithm.

TestSURF() has the check at the top of the function:

func TestSURF(t *testing.T) {
	testNonFree := os.Getenv("OPENCV_ENABLE_NONFREE")
	if testNonFree == "" {
		t.Skip("Skipping SURF test since OPENCV_ENABLE_NONFREE was not set")
	}

	img := gocv.IMRead("../images/face.jpg", gocv.IMReadGrayScale)
	if img.Empty() {
		t.Error("Invalid Mat in SURF test")
	}
	defer img.Close()
    ...

However TestSIFT() is missing that check:

func TestSIFT(t *testing.T) {
	img := IMRead("./images/face.jpg", IMReadGrayScale)
	if img.Empty() {
		t.Error("Invalid Mat in SIFT test")
	}
	defer img.Close()
    ...

I identified the issue in the dev branch and I couldn't find an existing PR or issue that mentions this. It's also possible that I've overlooked something and this is either already being addressed, or has been omitted intentionally.

Steps to Reproduce

N/A

Your Environment

N/A