Test and harden selection in `loc.*()` functions

[machow]

There are 5 loc functions that can do selection based on the data. While they're wired up in #341, we should do more thorough testing of them:

• [x] : loc.row_groups(groups=...)
• [x] : loc.stub(rows=...)
• [x] : loc.column_labels(columns=...)
• [ ] : loc.column_spanners(columns=...)
• [x] : loc.body()