Add raise_if_computes for testing

[tomwhite]

Fixes #310

This is useful for xarray testing.

I had hoped to be able to implement this by writing an executor that raises when its execute_dag method is called - but that doesn't work. The problem is that the second part of this test fails, since b's spec object contains the raise-if-computes executor since it was created within the context manager. Even though compute is called outside the context manager, the executor is still set on the spec, so that is what is used.

def test_raise_if_computes():
    # shouldn't raise since compute has not been called
    with raise_if_computes():
        a = xp.ones((3, 3), chunks=(2, 2))
        b = xp.negative(a)

    # shouldn't raise since we are outside the context manager
    assert_array_equal(b.compute(), -np.ones((3, 3)))

I'm not sure that changing this is the right thing to do, hence this PR which does things a bit more directly.

[TomNicholas]

This is definitely needed, but the current solution seems a little ugly - modifying the business logic just for testing purposes.

Could you pull a trick using a dedicated callback instead? How does dask do this?

Another failure mode that it would be nice to check for automatically is cubed-xarray accidentally wrapping a cubed array with a dask array. Isinstance checks would catch that but that has to be manually added to each test then. See Raphael's rechunking issue for an example.

[tomwhite]

I'm not too fond of this either, but it will probably need some more fundamental changes to how the spec/config is used to make the special executor/callback approach work. That would be a better solution though.

[tomwhite]

Closing this for the time being