harmonicenergytest.cc: Only test with points that are mutually close

Previously, this test used a situation with geodesic interpolation
between two points on the sphere that are almost antipodal.
This made the TargetSpaceRiemannianTRSolver crash, because the
weighted sum of squared distances can have indefinite second
derivatives in such a situation.  In principle, the
TargetSpaceRiemannianTRSolver should handle this, but nowadays
it is not actually a TR solver anymore, but a plain local
Newton method.  Maybe I should revert back to trust-region.

I added a comment to targetspacetrsolver.cc that explains the
situation a little.