Replaced location IDs with CSS to make tests flexible

Slugifying the prod Demo course broke some tests that relied on
specific problem IDs. Changing these IDs to CSS lets the tests work on
both prod and stage.
Yes, it'll actually work without .capa_inputtype too (as below, in line 51). I left them in here as a reminder to myself of how the CSS works from general –> narrow, in case later tests require the more thorough syntax.