TST: fix some mpl warnings
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

This change fixes `UserWarning` being emitted in tests calling `autocorrelation_plot`, `andrews_curves`, and `radviz`. 

`UserWarning` was caused by calling `plt.gca(xlim=..., ylim=...)` on a potentially already existing axis, so `matplotlib` warns about this forcing new axis creation. Solution was just to explicitly use `set_xlim` and `set_ylim` instead.

Does it make sense to also add to the relevant tests an assertion that no UserWarning is emitted to guarantee the new behavior?