Modify test_equal for values close to zero

I noticed the UnitTestTransform3D test failed using the random seed 1480544620. On closer inspection, I found that the issue was with the comparison of two numbers close to 0. The numbers were just above the threshold, but their difference was not quite enough to make the ratio below the threshold.

After reviewing some other floating point comparisons, they seem to be more forgiving of numbers close to 0. Thus, I changed this comparison to pass if the difference between the numbers was below the threshold.