OBJ vertex color related tests were not producing identical results
across various platforms, primarily due to sRGB<->Linear color space
conversions.
While D15193 has just made the color space conversion accuracy match
much closer between platforms, it's still not 100% the same.
This change reduces the amount of decimal places used for exporting
vertex colors, to 4 digits (down from 6). Vertex normals were
already always printed with 4 digits, and colors are conceptually
similar (usually 0..1 range etc.).
This makes the vertex color tests pass again, so re-enable them
after adjusting to 4 decimals expectations.
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