normal_vectors test is flaky

[ahoenselaar]

Repeated execution of the normal_vectors test will eventually result in test failure:

[...]
cone, center = (0,0,0)
     radius 1.19231, height 1.26, axis (-0.193605, -0.981053, 0.0071893)
     radius2 0.662376
mean error angle for 50-pt formula = 9.33924 +/- 7.98156 degrees, max error = 56.9723 degrees

block, center = (0,0,0)
     size (0.648139,0.83646,0.964154)
     axes (-0.853273,-0.195433,-0.483457), (-0.700766,0.0751388,-0.709423), (0.515375,0.735343,-0.440067)
CHECK failure on line 92 of normal_vectors.c: couldn't get normal vector