MNT: Fix skewT CAPE/CIN test with matplotlib 3.2
MNT: Fix skewT CAPE/CIN test with matplotlib 3.2

This test was created before aspect ratio work, but merged after. We use
aspect ratio='auto' to keep the images the same across matplotlib
versions, but this test was missing it. It didn't matter until
matplotlib 3.2 came out.

ShawnMurd/MetPy,dopplershift/MetPy,Unidata/MetPy,Unidata/MetPy,dopplershift/MetPy