FIX Remove code left by mistake during rebasing breaking normal orientation picking

This PR fixes a bug I, unfortunately, introduced while rebasing the orientation picking branch. The bug caused `W` to be sliced when picking normal orientation even though the slicing was already done for the gain matrix.

I only noticed the bug by running the examples (while building the docs). The tests didn't pick it up, since neither of the tests even accessed the resulting source estimate data. So I decided to bring back the tests I first wrote, which aren't very good (I removed them in the last PR...), but are better than nothing and would detect at least errors like the one fixed in this PR. WDYT?