`Variable` test categories

[keewis]

Here's a list of the test categories I think make sense:

• creation (simple test classes for creation and reduction #2)
• reduction (numeric, boolean, cumulative, order, order index) (simple test classes for creation and reduction #2)
• indexing
  • scalar, slice, integer array (indexing tests #14)
  • boolean array
• reshaping / coarsen
  • stack / unstack
  • expand_dims / squeeze
  • permute_dims / transpose
  • coarsen_reshape / coarsen
  • roll / shift
  • pad
• concat
• broadcasting
• elementwise / arithmetic
• chunked arrays