Remove Dask single-threaded setting in tests (#7489)

Co-authored-by: Joe Hamman <[email protected]>

Author: jrbourbeau

xarray/tests/__init__.py

@@ -101,8 +101,6 @@ def _importorskip(
 if has_dask:
     import dask
 
-    dask.config.set(scheduler="single-threaded")
-
 
 class CountingScheduler:
     """Simple dask scheduler counting the number of computes.