Horne extraction test for non-flat trace is unstable

[hpparvi]

The last assert_quantity_allcose in test_horne_non_flat_trace test checking that the Horne extraction from a flat trace equals the one from a non-flat trace fails occasionally. This seems to be rather rare and random issue, and difficult to reproduce, but it would be very good to understand whether this is a problem in the test design or Horne extraction itself. An easy fix would be to increase the assertion tolerances, but we shouldn't do this before we understand what is causing the discrepancy.