send vorticity to devnull

this is more representative of what tom actually wanted to do (write to zarr). we don't need to keep the whole thing in memory.

tests/workloads/test_array.py

@@ -123,7 +123,7 @@ def pad_rechunk(arr):
     vp = pad_rechunk(v)
     result = dx[..., None] * up - dy[..., None] * vp
 
-    wait(result, small_client, 10 * 60)
+    wait(arr_to_devnull(result), small_client, 10 * 60)
 
 
 def test_double_diff(small_client):