Add property testing to convolution

We have essentially no tests of convolution with asymmetric kernels, which means that if we had confused the cross-correlation definition with the convolution definition, we wouldn't have any tests to tell us. There are some nice property tests we can add: https://en.wikipedia.org/wiki/Convolution#Algebraic_properties

Start by locating the convolution implementation and its existing tests in the Astropy repository. Use the issue's Wikipedia link to identify relevant algebraic properties and focus on asymmetric kernels. Done means the properties are covered by tests that distinguish convolution from cross-correlation.

Written by the indexing model from the issue text.