Add conftest.py to allow skipping slow test.
Add conftest.py to allow skipping slow test.

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