Added unit test for select on Layout
Added unit test for select on Layout

Ensures that selecting HoloMap keys on a Layout containing
HoloMaps with non-overlapping keys works.

basnijholt/holoviews,basnijholt/holoviews,basnijholt/holoviews,ioam/holoviews,mjabri/holoviews,mjabri/holoviews,mjabri/holoviews,vascotenner/holoviews,ioam/holoviews,ioam/holoviews,vascotenner/holoviews,vascotenner/holoviews