TST: Debug flaky plotting test
Maybe closes https://github.com/pandas-dev/pandas/issues/19924. I can't reproduce this locally so who knows.

A while back I went through and added `plt.subplots()`  and explicit `ax=ax` to a bunch of the plotting tests. I apparently missed this one. It seemed to help, but they're flaky tests so who knows.

Also chanegd `idx1.equals(idx2)` to `tm.assert_frame_equal` so we get a better error message when it does fail.