If the selection stack only has one element (which it normally does), then
`design->pop_selection()` automatically resets to the default full selection.
This is a problem for `select [-none | -clear]` which were trying to replace the
current selection, but because the pop added an extra element when the `execute`
returned, the extra selection (the one we actually wanted) gets popped too. So
instead, reassign `design->selection()` in the same way as if we called `select
[selection]`.
Also adds selection stack tests, and removes the accidentally-committed
`boxes_dummy.ys`.
