add xarray+h5coro engine to suite of tests

[JessicaS11]

Looking at some of the graphs (presented today/yesterday by @andypbarrett) for h5py vs xarray vs h5coro vs kerchunk plots showing time to open hdf5 files (original, metadata reformatted, etc.). @betolink @jpswinski Does it make sense to add an xarray+h5coro engine case here?