Command line tests depend on proper installation

Command line tests assume that the executables `dask-scheduler` and `dask-worker` are on the path. This can cause confusion if developers don't install before testing, or if version confusion stops the current version from taking precedence. I've found myself having to do the following to get consistent behavior:

```python
pip uninstall distributed # repeat as necessary
python setup.py install
```

What's the right thing to do here? Should we instead be testing with `python -m distributed.cli.dask_scheduler...`?

cc @pitrou @mariusvniekerk