Fix test broken by adding heartbeat
Fix test broken by adding heartbeat

The test was not expecting the Scheduler.workers dict ot contain
'last-seen' times.

jayhetee/dask,cowlicks/dask,gameduell/dask,mikegraham/dask,jayhetee/dask,jakirkham/dask,simudream/dask,jcrist/dask,blaze/dask,jakirkham/dask,pombredanne/dask,PhE/dask,blaze/dask,ContinuumIO/dask,mrocklin/dask,ssanderson/dask,dask/dask,freeman-lab/dask,mraspaud/dask,mraspaud/dask,PhE/dask,clarkfitzg/dask,simudream/dask,mrocklin/dask,ContinuumIO/dask,jcrist/dask,chrisbarber/dask,dask/dask,wiso/dask,clarkfitzg/dask,pombredanne/dask,vikhyat/dask,ssanderson/dask,freeman-lab/dask,wiso/dask,vikhyat/dask,cpcloud/dask