sns.catplot() returns a FacetGrid object. You need a matplotlib Figure object for the tests to work correctly and to activate the Axes object. I accomplished this by using
graph = sns.catplot(...)
# Define fig for testing.
fig = graph.fig
# Call legend to make the legend active for testing.
ax = fig.axes[0].legend()
I mucked around reading the testing module to figure this out.
Posting a link to your repl.it project may also be helpful.