Add tests for colortables registry API.
Add tests for colortables registry API.

Unidata/MetPy,ahaberlie/MetPy,dopplershift/MetPy,ShawnMurd/MetPy,jrleeman/MetPy,ahaberlie/MetPy,jrleeman/MetPy,deeplycloudy/MetPy,Unidata/MetPy,ahill818/MetPy,dopplershift/MetPy