test(rxSortableColumn): All-column functions

Useful for when you need all column names, or their sorts by table.

Includes some refactoring for column sorting, which now gets reused.

On my machine I'm not getting any "more than one element found" for the
rxSortableColumn tests, but in headless environments it's almost
like...there's two sets of columns somehow? I'm not sure.

I've noticed that the first test doesn't seem to complain, but
afterwards, it starts to see multiple sets of columns. I have a
suspicion that there's some kind of odd behavior that happens after the
first interaction with the columns, so I've carefully arranged these
tests so that the new ones come first, which don't manipulate columns.