test(aliby): fix non-iterable in pytest parametrize

WHY IS THIS CHANGE NEEDED?:
- https://git.ecdf.ed.ac.uk/swain-lab/aliby/aliby/-/jobs/56757 failed
  because TypeError raised: 'int' object is not iterable

HOW DOES THE CHANGE SOLVE THE PROBLEM?:
- @pytest.mark.parametrize() takes an iterable, e.g. a list -- I forgot
  about it.  fixed it by putting the values in a list.

REFERENCES:
- issue #53

tests/aliby/pipeline/test_image.py

@@ -14,9 +14,9 @@ sample_da = da.reshape(
 )
 
 
-@pytest.mark.parametrize("sample_da", sample_da)
-@pytest.mark.parametrize("dim", 2)
-@pytest.mark.parametrize("n_empty_slices", 4)
+@pytest.mark.parametrize("sample_da", [sample_da])
+@pytest.mark.parametrize("dim", [2])
+@pytest.mark.parametrize("n_empty_slices", [4])
 def test_pad_array(sample_da, dim, n_empty_slices):
     """Test ImageDummy.pad_array() method"""
     # create object