test(aliby): fix argument logic in test_image

WHY IS THIS CHANGE NEEDED?:
- https://git.ecdf.ed.ac.uk/swain-lab/aliby/aliby/-/jobs/56762 failed
  because TypeError: compute() takes 1 positional argument but 2 were given

HOW DOES THE CHANGE SOLVE THE PROBLEM?:
- corrected a stupid typo: compute() is a function that takes no
  arguments that gives out an array, and i want to index that array

REFERENCES:
- issue #53

tests/aliby/pipeline/test_image.py

@@ -34,6 +34,6 @@ def test_pad_array(sample_da, dim, n_empty_slices):
     ]
 
     # Checks that original image array is there and is at the last index
-    assert padded_da.compute(ix) == sample_da
+    assert padded_da.compute()[ix] == sample_da
     # Checks that the additional axis is extended correctly
     assert padded_da.compute.shape[dim] == n_empty_slices + 1