Tests: Lower Sculpt Dyntopo mesh size to 250000 in performance test

This commit lowers the size of the mesh from approximately 2 million
verts to 250 thousand verts. This brings the graph more in line with the
mesh and multires usecase, and is more helpful in measuring and
detecting real performance issues.

While the upper end of dyntopo can certainly be massively improved,
strokes that take in the seconds to complete are already unusable from a
user perspective,

tests/performance/tests/sculpt.py

@@ -72,7 +72,7 @@ def prepare_sculpt_scene(context: any, mode: SculptMode):
     elif mode == SculptMode.MULTIRES:
         size = 150
     elif mode == SculptMode.DYNTOPO:
-        size = 1500
+        size = 500
     else:
         raise NotImplementedError