Added new unit tests for overlay and layout composition
Added new unit tests for overlay and layout composition

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