Added tests for active_link_ids.
Added tests for active_link_ids.

RondaStrauch/landlab,landlab/landlab,amandersillinois/landlab,RondaStrauch/landlab,SiccarPoint/landlab,Carralex/landlab,decvalts/landlab,laijingtao/landlab,RondaStrauch/landlab,ManuSchmi88/landlab,landlab/landlab,decvalts/landlab,landlab/landlab,SiccarPoint/landlab,laijingtao/landlab,cmshobe/landlab,cmshobe/landlab,ManuSchmi88/landlab,cmshobe/landlab,csherwood-usgs/landlab,amandersillinois/landlab,Carralex/landlab,ManuSchmi88/landlab,csherwood-usgs/landlab,Carralex/landlab