BUG: Fix spurious AttributeError in testing helper.
BUG: Fix spurious AttributeError in testing helper.

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