test_png_with_16_bits_per_channel: Skip test with OpenCV 2
test_png_with_16_bits_per_channel: Skip test with OpenCV 2

`cv2.imread("tests/uint16.png", cv2.IMREAD_UNCHANGED)` behaves
differently on OpenCV 2 vs 3:

| OpenCV version | Shape         |
|----------------|---------------|
| 2.4.9.1        | (301, 349)    |
| 3.2.0          | (301, 349, 4) |

Maybe 2.x doesn't support grayscale+alpha, and 3.x supports it by
converting greyscale to BGR.

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester