add test for indexing with integer arrays

I had a try at solving issue #5006, but it does not look as trivial to me as @jreback suggested. ;-)

I added a test for indexing with int-typed arrays containing single indices, which currently fails.  I spent some time reading test_indexing, because my hunch is that it could be better integrated with the existing tests (e.g. using check_result), but anyhow - any test is better than none, I guess.

The failing test goes through `_iLocIndexer._getitem_axis()` -> `_NDFrameIndexer._get_loc()` -> `Series._ixs()` -> `index.get_value_at()` -> `util.get_value_at()`, but I don't see the point where that should've been different right now.

Sorry, but I lack time for solving this at the moment.