Avoid integer overflow in arc test.  Fixes #1918

I took a look at #1918 (also #1949 which is a duplicate).   What was happening was that the large input values in the arc.t test polyline were being scaled by Slic3r::SCALING_FACTOR in the scale_new constructor, after which they exceeded LONG_MAX; they were then truncated to longs when the final Slic3r::Polyline class was constructed.   As a result the actual coordinates were lost by the time the Polyline->scale in arc.t was attempted.